Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
Concerning the first, three things occur to be considered: first, the fitness of the Incarnation; secondly, the mode of union of the Word Incarnate; thirdly, what follows this union.
Under the first head there are six points of inquiry:
(1) Whether it is fitting for God to become incarnate?
(2) Whether it was necessary for the restoration of the human race?
(3) Whether if there had been no sin God would have become incarnate?
(4) Whether He became incarnate to take away original sin rather than actual?
(5) Whether it was fitting for God to become incarnate from the beginning of the world?
(6) Whether His Incarnation ought to have been deferred to the end of the world?
Objection 1: It would seem that it was not fitting for God to become incarnate. Since God from all eternity is the very essence of goodness, it was best for Him to be as He had been from all eternity. But from all eternity He had been without flesh. Therefore it was most fitting for Him not to be united to flesh. Therefore it was not fitting for God to become incarnate.
Objection 2: Further, it is not fitting to unite things that are infinitely apart, even as it would not be a fitting union if one were "to paint a figure in which the neck of a horse was joined to the head of a man" [*Horace, Ars. Poet., line 1]. But God and flesh are infinitely apart; since God is most simple, and flesh is most composite---especially human flesh. Therefore it was not fitting that God should be united to human flesh.
Objection 3: Further, a body is as distant from the highest spirit as evil is from the highest good. But it was wholly unfitting that God, Who is the highest good, should assume evil. Therefore it was not fitting that the highest uncreated spirit should assume a body.
Objection 4: Further, it is not becoming that He Who surpassed the greatest things should be contained in the least, and He upon Whom rests the care of great things should leave them for lesser things. But God---Who takes care of the whole world---the whole universe of things cannot contain. Therefore it would seem unfitting that "He should be hid under the frail body of a babe in swathing bands, in comparison with Whom the whole universe is accounted as little; and that this Prince should quit His throne for so long, and transfer the government of the whole world to so frail a body," as Volusianus writes to Augustine (Ep. cxxxv).
On the contrary, It would seem most fitting that by visible things the invisible things of God should be made known; for to this end was the whole world made, as is clear from the word of the Apostle (Rom. 1:20): "For the invisible things of God . . . are clearly seen, being understood by the things that are made." But, as Damascene says (De Fide Orth. iii, 1), by the mystery of the Incarnation are made known at once the goodness, the wisdom, the justice, and the power or might of God---"His goodness, for He did not despise the weakness of His own handiwork; His justice, since, on man's defeat, He caused the tyrant to be overcome by none other than man, and yet He did not snatch men forcibly from death; His wisdom, for He found a suitable discharge for a most heavy debt; His power, or infinite might, for there is nothing greater than for God to become incarnate . . ."
I answer that, To each things, that is befitting which belongs to it by reason of its very nature; thus, to reason befits man, since this belongs to him because he is of a rational nature. But the very nature of God is goodness, as is clear from Dionysius (Div. Nom. i). Hence, what belongs to the essence of goodness befits God. But it belongs to the essence of goodness to communicate itself to others, as is plain from Dionysius (Div. Nom. iv). Hence it belongs to the essence of the highest good to communicate itself in the highest manner to the creature, and this is brought about chiefly by "His so joining created nature to Himself that one Person is made up of these three---the Word, a soul and flesh," as Augustine says (De Trin. xiii). Hence it is manifest that it was fitting that God should become incarnate.
Reply to Objection 1: The mystery of the Incarnation was not completed through God being changed in any way from the state in which He had been from eternity, but through His having united Himself to the creature in a new way, or rather through having united it to Himself. But it is fitting that a creature which by nature is mutable, should not always be in one way. And therefore, as the creature began to be, although it had not been before, so likewise, not having been previously united to God in Person, it was afterwards united to Him.
Reply to Objection 2: To be united to God in unity of person was not fitting to human flesh, according to its natural endowments, since it was above its dignity; nevertheless, it was fitting that God, by reason of His infinite goodness, should unite it to Himself for man's salvation.
Reply to Objection 3: Every mode of being wherein any creature whatsoever differs from the Creator has been established by God's wisdom, and is ordained to God's goodness. For God, Who is uncreated, immutable, and incorporeal, produced mutable and corporeal creatures for His own goodness. And so also the evil of punishment was established by God's justice for God's glory. But evil of fault is committed by withdrawing from the art of the Divine wisdom and from the order of the Divine goodness. And therefore it could be fitting to God to assume a nature created, mutable, corporeal, and subject to penalty, but it did not become Him to assume the evil of fault.
Reply to Objection 4: As Augustine replies (Ep. ad Volusian. cxxxvii): "The Christian doctrine nowhere holds that God was so joined to human flesh as either to desert or lose, or to transfer and as it were, contract within this frail body, the care of governing the universe. This is the thought of men unable to see anything but corporeal things . . . God is great not in mass, but in might. Hence the greatness of His might feels no straits in narrow surroundings. Nor, if the passing word of a man is heard at once by many, and wholly by each, is it incredible that the abiding Word of God should be everywhere at once?" Hence nothing unfitting arises from God becoming incarnate.
Objection 1: It would seem that it was not necessary for the reparation of the human race that the Word of God should become incarnate. For since the Word of God is perfect God, as has been said (FP, Q[4], AA[1],2), no power was added to Him by the assumption of flesh. Therefore, if the incarnate Word of God restored human nature. He could also have restored it without assuming flesh.
Objection 2: Further, for the restoration of human nature, which had fallen through sin, nothing more is required than that man should satisfy for sin. Now man can satisfy, as it would seem, for sin; for God cannot require from man more than man can do, and since He is more inclined to be merciful than to punish, as He lays the act of sin to man's charge, so He ought to credit him with the contrary act. Therefore it was not necessary for the restoration of human nature that the Word of God should become incarnate.
Objection 3: Further, to revere God pertains especially to man's salvation; hence it is written (Mal. 1:6): "If, then, I be a father, where is my honor? and if I be a master, where is my fear?" But men revere God the more by considering Him as elevated above all, and far beyond man's senses, hence (Ps. 112:4) it is written: "The Lord is high above all nations, and His glory above the heavens"; and farther on: "Who is as the Lord our God?" which pertains to reverence. Therefore it would seem unfitting to man's salvation that God should be made like unto us by assuming flesh.
On the contrary, What frees the human race from perdition is necessary for the salvation of man. But the mystery of the Incarnation is such; according to Jn. 3:16: "God so loved the world as to give His only-begotten Son, that whosoever believeth in Him may not perish, but may have life everlasting." Therefore it was necessary for man's salvation that God should become incarnate.
I answer that, A thing is said to be necessary for a certain end in two ways. First, when the end cannot be without it; as food is necessary for the preservation of human life. Secondly, when the end is attained better and more conveniently, as a horse is necessary for a journey. In the first way it was not necessary that God should become incarnate for the restoration of human nature. For God with His omnipotent power could have restored human nature in many other ways. But in the second way it was necessary that God should become incarnate for the restoration of human nature. Hence Augustine says (De Trin. xii, 10): "We shall also show that other ways were not wanting to God, to Whose power all things are equally subject; but that there was not a more fitting way of healing our misery."
Now this may be viewed with respect to our "furtherance in good." First, with regard to faith, which is made more certain by believing God Himself Who speaks; hence Augustine says (De Civ. Dei xi, 2): "In order that man might journey more trustfully toward the truth, the Truth itself, the Son of God, having assumed human nature, established and founded faith." Secondly, with regard to hope, which is thereby greatly strengthened; hence Augustine says (De Trin. xiii): "Nothing was so necessary for raising our hope as to show us how deeply God loved us. And what could afford us a stronger proof of this than that the Son of God should become a partner with us of human nature?" Thirdly, with regard to charity, which is greatly enkindled by this; hence Augustine says (De Catech. Rudib. iv): "What greater cause is there of the Lord's coming than to show God's love for us?" And he afterwards adds: "If we have been slow to love, at least let us hasten to love in return." Fourthly, with regard to well-doing, in which He set us an example; hence Augustine says in a sermon (xxii de Temp.): "Man who might be seen was not to be followed; but God was to be followed, Who could not be seen. And therefore God was made man, that He Who might be seen by man, and Whom man might follow, might be shown to man." Fifthly, with regard to the full participation of the Divinity, which is the true bliss of man and end of human life; and this is bestowed upon us by Christ's humanity; for Augustine says in a sermon (xiii de Temp.): "Go was made man, that man might be made God."
So also was this useful for our "withdrawal from evil." First, because man is taught by it not to prefer the devil to himself, nor to honor him who is the author of sin; hence Augustine says (De Trin. xiii, 17): "Since human nature is so united to God as to become one person, let not these proud spirits dare to prefer themselves to man, because they have no bodies." Secondly, because we are thereby taught how great is man's dignity, lest we should sully it with sin; hence Augustine says (De Vera Relig. xvi): "God has proved to us how high a place human nature holds amongst creatures, inasmuch as He appeared to men as a true man." And Pope Leo says in a sermon on the Nativity (xxi): "Learn, O Christian, thy worth; and being made a partner of the Divine nature, refuse to return by evil deeds to your former worthlessness." Thirdly, because, "in order to do away with man's presumption, the grace of God is commended in Jesus Christ, though no merits of ours went before," as Augustine says (De Trin. xiii, 17). Fourthly, because "man's pride, which is the greatest stumbling-block to our clinging to God, can be convinced and cured by humility so great," as Augustine says in the same place. Fifthly, in order to free man from the thraldom of sin, which, as Augustine says (De Trin. xiii, 13), "ought to be done in such a way that the devil should be overcome by the justice of the man Jesus Christ," and this was done by Christ satisfying for us. Now a mere man could not have satisfied for the whole human race, and God was not bound to satisfy; hence it behooved Jesus Christ to be both God and man. Hence Pope Leo says in the same sermon: "Weakness is assumed by strength, lowliness by majesty, mortality by eternity, in order that one and the same Mediator of God and men might die in one and rise in the other---for this was our fitting remedy. Unless He was God, He would not have brought a remedy; and unless He was man, He would not have set an example."
And there are very many other advantages which accrued, above man's apprehension.
Reply to Objection 1: This reason has to do with the first kind of necessity, without which we cannot attain to the end.
Reply to Objection 2: Satisfaction may be said to be sufficient in two ways---first, perfectly, inasmuch as it is condign, being adequate to make good the fault committed, and in this way the satisfaction of a mere man cannot be sufficient for sin, both because the whole of human nature has been corrupted by sin, whereas the goodness of any person or persons could not be made up adequately for the harm done to the whole of the nature; and also because a sin committed against God has a kind of infinity from the infinity of the Divine majesty, because the greater the person we offend, the more grievous the offense. Hence for condign satisfaction it was necessary that the act of the one satisfying should have an infinite efficiency, as being of God and man. Secondly, man's satisfaction may be termed sufficient, imperfectly---i.e. in the acceptation of him who is content with it, even though it is not condign, and in this way the satisfaction of a mere man is sufficient. And forasmuch as every imperfect presupposes some perfect thing, by which it is sustained, hence it is that satisfaction of every mere man has its efficiency from the satisfaction of Christ.
Reply to Objection 3: By taking flesh, God did not lessen His majesty; and in consequence did not lessen the reason for reverencing Him, which is increased by the increase of knowledge of Him. But, on the contrary, inasmuch as He wished to draw nigh to us by taking flesh, He greatly drew us to know Him.
Objection 1: It would seem that if man had not sinned, God would still have become incarnate. For the cause remaining, the effect also remains. But as Augustine says (De Trin. xiii, 17): "Many other things are to be considered in the Incarnation of Christ besides absolution from sin"; and these were discussed above (A[2]). Therefore if man had not sinned, God would have become incarnate.
Objection 2: Further, it belongs to the omnipotence of the Divine power to perfect His works, and to manifest Himself by some infinite effect. But no mere creature can be called an infinite effect, since it is finite of its very essence. Now, seemingly, in the work of the Incarnation alone is an infinite effect of the Divine power manifested in a special manner by which power things infinitely distant are united, inasmuch as it has been brought about that man is God. And in this work especially the universe would seem to be perfected, inasmuch as the last creature---viz. man---is united to the first principle---viz. God. Therefore, even if man had not sinned, God would have become incarnate.
Objection 3: Further, human nature has not been made more capable of grace by sin. But after sin it is capable of the grace of union, which is the greatest grace. Therefore, if man had not sinned, human nature would have been capable of this grace; nor would God have withheld from human nature any good it was capable of. Therefore, if man had not sinned, God would have become incarnate.
Objection 4: Further, God's predestination is eternal. But it is said of Christ (Rom. 1:4): "Who was predestined the Son of God in power." Therefore, even before sin, it was necessary that the Son of God should become incarnate, in order to fulfil God's predestination.
Objection 5: Further, the mystery of the Incarnation was revealed to the first man, as is plain from Gn. 2:23. "This now is bone of my bones," etc. which the Apostle says is "a great sacrament . . . in Christ and in the Church," as is plain from Eph. 5:32. But man could not be fore-conscious of his fall, for the same reason that the angels could not, as Augustine proves (Gen. ad lit. xi, 18). Therefore, even if man had not sinned, God would have become incarnate.
On the contrary, Augustine says (De Verb. Apost. viii, 2), expounding what is set down in Lk. 19:10, "For the Son of Man is come to seek and to save that which was lost"; "Therefore, if man had not sinned, the Son of Man would not have come." And on 1 Tim. 1:15, "Christ Jesus came into this world to save sinners," a gloss says, "There was no cause of Christ's coming into the world, except to save sinners. Take away diseases, take away wounds, and there is no need of medicine."
I answer that, There are different opinions about this question. For some say that even if man had not sinned, the Son of Man would have become incarnate. Others assert the contrary, and seemingly our assent ought rather to be given to this opinion.
For such things as spring from God's will, and beyond the creature's due, can be made known to us only through being revealed in the Sacred Scripture, in which the Divine Will is made known to us. Hence, since everywhere in the Sacred Scripture the sin of the first man is assigned as the reason of the Incarnation, it is more in accordance with this to say that the work of the Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, the Incarnation would not have been. And yet the power of God is not limited to this; even had sin not existed, God could have become incarnate.
Reply to Objection 1: All the other causes which are assigned in the preceding article have to do with a remedy for sin. For if man had not sinned, he would have been endowed with the light of Divine wisdom, and would have been perfected by God with the righteousness of justice in order to know and carry out everything needful. But because man, on deserting God, had stooped to corporeal things, it was necessary that God should take flesh, and by corporeal things should afford him the remedy of salvation. Hence, on Jn. 1:14, "And the Word was made flesh," St. Augustine says (Tract. ii): "Flesh had blinded thee, flesh heals thee; for Christ came and overthrew the vices of the flesh."
Reply to Objection 2: The infinity of Divine power is shown in the mode of production of things from nothing. Again, it suffices for the perfection of the universe that the creature be ordained in a natural manner to God as to an end. But that a creature should be united to God in person exceeds the limits of the perfection of nature.
Reply to Objection 3: A double capability may be remarked in human nature: one, in respect of the order of natural power, and this is always fulfilled by God, Who apportions to each according to its natural capability; the other in respect to the order of the Divine power, which all creatures implicitly obey; and the capability we speak of pertains to this. But God does not fulfil all such capabilities, otherwise God could do only what He has done in creatures, and this is false, as stated above (FP, Q[105], A[6]). But there is no reason why human nature should not have been raised to something greater after sin. For God allows evils to happen in order to bring a greater good therefrom; hence it is written (Rom. 5:20): "Where sin abounded, grace did more abound." Hence, too, in the blessing of the Paschal candle, we say: "O happy fault, that merited such and so great a Redeemer!"
Reply to Objection 4: Predestination presupposes the foreknowledge of future things; and hence, as God predestines the salvation of anyone to be brought about by the prayers of others, so also He predestined the work of the Incarnation to be the remedy of human sin.
Reply to Objection 5: Nothing prevents an effect from being revealed to one to whom the cause is not revealed. Hence, the mystery of the Incarnation could be revealed to the first man without his being fore-conscious of his fall. For not everyone who knows the effect knows the cause.
Objection 1: It would seem that God became incarnate as a remedy for actual sins rather than for original sin. For the more grievous the sin, the more it runs counter to man's salvation, for which God became incarnate. But actual sin is more grievous than original sin; for the lightest punishment is due to original sin, as Augustine says (Contra Julian. v, 11). Therefore the Incarnation of Christ is chiefly directed to taking away actual sins.
Objection 2: Further, pain of sense is not due to original sin, but merely pain of loss, as has been shown (FS, Q[87], A[5]). But Christ came to suffer the pain of sense on the Cross in satisfaction for sins---and not the pain of loss, for He had no defect of either the beatific vision or fruition. Therefore He came in order to take away actual sin rather than original sin.
Objection 3: Further, as Chrysostom says (De Compunctione Cordis ii, 3): "This must be the mind of the faithful servant, to account the benefits of his Lord, which have been bestowed on all alike, as though they were bestowed on himself alone. For as if speaking of himself alone, Paul writes to the Galatians 2:20: 'Christ . . . loved me and delivered Himself for me.'" But our individual sins are actual sins; for original sin is the common sin. Therefore we ought to have this conviction, so as to believe that He has come chiefly for actual sins.
On the contrary, It is written (Jn. 1:29): "Behold the Lamb of God, behold Him Who taketh away the sins [Vulg.: 'sin'] of the world."
I answer that, It is certain that Christ came into this world not only to take away that sin which is handed on originally to posterity, but also in order to take away all sins subsequently added to it; not that all are taken away (and this is from men's fault, inasmuch as they do not adhere to Christ, according to Jn. 3:19: "The light is come into the world, and men loved darkness rather than the light"), but because He offered what was sufficient for blotting out all sins. Hence it is written (Rom. 5:15-16): "But not as the offense, so also the gift . . . For judgment indeed was by one unto condemnation, but grace is of many offenses unto justification."
Moreover, the more grievous the sin, the more particularly did Christ come to blot it out. But "greater" is said in two ways: in one way "intensively," as a more intense whiteness is said to be greater, and in this way actual sin is greater than original sin; for it has more of the nature of voluntary, as has been shown (FS, Q[81], A[1]). In another way a thing is said to be greater "extensively," as whiteness on a greater superficies is said to be greater; and in this way original sin, whereby the whole human race is infected, is greater than any actual sin, which is proper to one person. And in this respect Christ came principally to take away original sin, inasmuch as "the good of the race is a more Divine thing than the good of an individual," as is said Ethic. i, 2.
Reply to Objection 1: This reason looks to the intensive greatness of sin.
Reply to Objection 2: In the future award the pain of sense will not be meted out to original sin. Yet the penalties, such as hunger, thirst, death, and the like, which we suffer sensibly in this life flow from original sin. And hence Christ, in order to satisfy fully for original sin, wished to suffer sensible pain, that He might consume death and the like in Himself.
Reply to Objection 3: Chrysostom says (De Compunctione Cordis ii, 6): "The Apostle used these words, not as if wishing to diminish Christ's gifts, ample as they are, and spreading throughout the whole world, but that he might account himself alone the occasion of them. For what does it matter that they are given to others, if what are given to you are as complete and perfect as if none of them were given to another than yourself?" And hence, although a man ought to account Christ's gifts as given to himself, yet he ought not to consider them not to be given to others. And thus we do not exclude that He came to wipe away the sin of the whole nature rather than the sin of one person. But the sin of the nature is as perfectly healed in each one as if it were healed in him alone. Hence, on account of the union of charity, what is vouchsafed to all ought to be accounted his own by each one.
Objection 1: It would seem that it was fitting that God should become incarnate in the beginning of the human race. For the work of the Incarnation sprang from the immensity of Divine charity, according to Eph. 2:4, 5: "But God (Who is rich in mercy), for His exceeding charity wherewith He loved us . . . even when we were dead in sins, hath quickened us together in Christ." But charity does not tarry in bringing assistance to a friend who is suffering need, according to Prov. 3:28: "Say not to thy friend: Go, and come again, and tomorrow I will give to thee, when thou canst give at present." Therefore God ought not to have put off the work of the Incarnation, but ought thereby to have brought relief to the human race from the beginning.
Objection 2: Further, it is written (1 Tim. 1:15): "Christ Jesus came into this world to save sinners." But more would have been saved had God become incarnate at the beginning of the human race; for in the various centuries very many, through not knowing God, perished in their sin. Therefore it was fitting that God should become incarnate at the beginning of the human race.
Objection 3: Further, the work of grace is not less orderly than the work of nature. But nature takes its rise with the more perfect, as Boethius says (De Consol. iii). Therefore the work of Christ ought to have been perfect from the beginning. But in the work of the Incarnation we see the perfection of grace, according to Jn. 1:14: "The Word was made flesh"; and afterwards it is added: "Full of grace and truth." Therefore Christ ought to have become incarnate at the beginning of the human race.
On the contrary, It is written (Gal. 4:4): "But when the fulness of the time was come, God sent His Son, made of a woman, made under the law": upon which a gloss says that "the fulness of the time is when it was decreed by God the Father to send His Son." But God decreed everything by His wisdom. Therefore God became incarnate at the most fitting time; and it was not fitting that God should become incarnate at the beginning of the human race.
I answer that, Since the work of the Incarnation is principally ordained to the restoration of the human race by blotting out sin, it is manifest that it was not fitting for God to become incarnate at the beginning of the human race before sin. For medicine is given only to the sick. Hence our Lord Himself says (Matt. 9:12, 13): "They that are in health need not a physician, but they that are ill . . . For I am not come to call the just, but sinners."
Nor was it fitting that God should become incarnate immediately after sin. First, on account of the manner of man's sin, which had come of pride; hence man was to be liberated in such a manner that he might be humbled, and see how he stood in need of a deliverer. Hence on the words in Gal. 3:19, "Being ordained by angels in the hand of a mediator," a gloss says: "With great wisdom was it so ordered that the Son of Man should not be sent immediately after man's fall. For first of all God left man under the natural law, with the freedom of his will, in order that he might know his natural strength; and when he failed in it, he received the law; whereupon, by the fault, not of the law, but of his nature, the disease gained strength; so that having recognized his infirmity he might cry out for a physician, and beseech the aid of grace."
Secondly, on account of the order of furtherance in good, whereby we proceed from imperfection to perfection. Hence the Apostle says (1 Cor. 15:46, 47): "Yet that was not first which is spiritual, but that which is natural; afterwards that which is spiritual . . . The first man was of the earth, earthy; the second man from heaven, heavenly."
Thirdly, on account of the dignity of the incarnate Word, for on the words (Gal. 4:4), "But when the fulness of the time was come," a gloss says: "The greater the judge who was coming, the more numerous was the band of heralds who ought to have preceded him."
Fourthly, lest the fervor of faith should cool by the length of time, for the charity of many will grow cold at the end of the world. Hence (Lk. 18:8) it is written: "But yet the Son of Man, when He cometh, shall He find think you, faith on earth?"
Reply to Objection 1: Charity does not put off bringing assistance to a friend: always bearing in mind the circumstances as well as the state of the persons. For if the physician were to give the medicine at the very outset of the ailment, it would do less good, and would hurt rather than benefit. And hence the Lord did not bestow upon the human race the remedy of the Incarnation in the beginning, lest they should despise it through pride, if they did not already recognize their disease.
Reply to Objection 2: Augustine replies to this (De Sex Quest. Pagan., Ep. cii), saying (Q[2]) that "Christ wished to appear to man and to have His doctrine preached to them when and where He knew those were who would believe in Him. But in such times and places as His Gospel was not preached He foresaw that not all, indeed, but many would so bear themselves towards His preaching as not to believe in His corporeal presence, even were He to raise the dead." But the same Augustine, taking exception to this reply in his book (De Perseverantia ix), says: "How can we say the inhabitants of Tyre and Sidon would not believe when such great wonders were wrought in their midst, or would not have believed had they been wrought, when God Himself bears witness that they would have done penance with great humility if these signs of Divine power had been wrought in their midst?" And he adds in answer (De Perseverantia xi): "Hence, as the Apostle says (Rom. 9:16), 'it is not of him that willeth nor of him that runneth, but of God that showeth mercy'; Who (succors whom He will of) those who, as He foresaw, would believe in His miracles if wrought amongst them, (while others) He succors not, having judged them in His predestination secretly yet justly. Therefore let us unshrinkingly believe His mercy to be with those who are set free, and His truth with those who are condemned." [*The words in brackets are not in the text of St. Augustine].
Reply to Objection 3: Perfection is prior to imperfection, both in time and nature, in things that are different (for what brings others to perfection must itself be perfect); but in one and the same, imperfection is prior in time though posterior in nature. And thus the eternal perfection of God precedes in duration the imperfection of human nature; but the latter's ultimate perfection in union with God follows.
Objection 1: It would seem that the work of the Incarnation ought to have been put off till the end of the world. For it is written (Ps. 91:11): "My old age in plentiful mercy"---i.e. "in the last days," as a gloss says. But the time of the Incarnation is especially the time of mercy, according to Ps. 101:14: "For it is time to have mercy on it." Therefore the Incarnation ought to have been put off till the end of the world.
Objection 2: Further, as has been said (A[5], ad 3), in the same subject, perfection is subsequent in time to imperfection. Therefore, what is most perfect ought to be the very last in time. But the highest perfection of human nature is in the union with the Word, because "in Christ it hath pleased the Father that all the fulness of the Godhead should dwell," as the Apostle says (Col. 1:19, and 2:9). Therefore the Incarnation ought to have been put off till the end of the world.
Objection 3: Further, what can be done by one ought not to be done by two. But the one coming of Christ at the end of the world was sufficient for the salvation of human nature. Therefore it was not necessary for Him to come beforehand in His Incarnation; and hence the Incarnation ought to have been put off till the end of the world.
On the contrary, It is written (Hab. 3:2): "In the midst of the years Thou shalt make it known." Therefore the mystery of the Incarnation which was made known to the world ought not to have been put off till the end of the world.
I answer that, As it was not fitting that God should become incarnate at the beginning of the world, so also it was not fitting that the Incarnation should be put off till the end of the world. And this is shown first from the union of the Divine and human nature. For, as it has been said (A[5], ad 3), perfection precedes imperfection in time in one way, and contrariwise in another way imperfection precedes perfection. For in that which is made perfect from being imperfect, imperfection precedes perfection in time, whereas in that which is the efficient cause of perfection, perfection precedes imperfection in time. Now in the work of the Incarnation both concur; for by the Incarnation human nature is raised to its highest perfection; and in this way it was not becoming that the Incarnation should take place at the beginning of the human race. And the Word incarnate is the efficient cause of the perfection of human nature, according to Jn. 1:16: "Of His fulness we have all received"; and hence the work of the Incarnation ought not to have been put off till the end of the world. But the perfection of glory to which human nature is to be finally raised by the Word Incarnate will be at the end of the world.
Secondly, from the effect of man's salvation; for, as is said Qq. Vet et Nov. Test., qu. 83, "it is in the power of the Giver to have pity when, or as much as, He wills. Hence He came when He knew it was fitting to succor, and when His boons would be welcome. For when by the feebleness of the human race men's knowledge of God began to grow dim and their morals lax, He was pleased to choose Abraham as a standard of the restored knowledge of God and of holy living; and later on when reverence grew weaker, He gave the law to Moses in writing; and because the gentiles despised it and would not take it upon themselves, and they who received it would not keep it, being touched with pity, God sent His Son, to grant to all remission of their sin and to offer them, justified, to God the Father." But if this remedy had been put off till the end of the world, all knowledge and reverence of God and all uprightness of morals would have been swept away from the earth.
Thirdly, this appears fitting to the manifestation of the Divine power, which has saved men in several ways---not only by faith in some future thing, but also by faith in something present and past.
Reply to Objection 1: This gloss has in view the mercy of God, which leads us to glory. Nevertheless, if it is referred to the mercy shown the human race by the Incarnation of Christ, we must reflect that, as Augustine says (Retract. i), the time of the Incarnation may be compared to the youth of the human race, "on account of the strength and fervor of faith, which works by charity"; and to old age---i.e. the sixth age---on account of the number of centuries, for Christ came in the sixth age. And although youth and old age cannot be together in a body, yet they can be together in a soul, the former on account of quickness, the latter on account of gravity. And hence Augustine says elsewhere (Qq. lxxxiii, qu. 44) that "it was not becoming that the Master by Whose imitation the human race was to be formed to the highest virtue should come from heaven, save in the time of youth." But in another work (De Gen. cont. Manich. i, 23) he says: that Christ came in the sixth age---i.e. in the old age---of the human race.
Reply to Objection 2: The work of the Incarnation is to be viewed not as merely the terminus of a movement from imperfection to perfection, but also as a principle of perfection to human nature, as has been said.
Reply to Objection 3: As Chrysostom says on Jn. 3:11, "For God sent not His Son into the world to judge the world" (Hom. xxviii): "There are two comings of Christ: the first, for the remission of sins; the second, to judge the world. For if He had not done so, all would have perished together, since all have sinned and need the glory of God." Hence it is plain that He ought not to have put off the coming in mercy till the end of the world.