Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We must now consider the species of superstition. We shall treat (1) Of the superstition which consists in giving undue worship to the true God; (2) Of the superstition of idolatry; (3) of divinatory superstition; (4) of the superstition of observances.
Under the first head there are two points of inquiry:
(1) Whether there can be anything pernicious in the worship of the true God?
(2) Whether there can be anything superfluous therein?
Objection 1: It would seem that there cannot be anything pernicious in the worship of the true God. It is written (Joel 2:32): "Everyone that shall call upon the name of the Lord shall be saved." Now whoever worships God calls upon His name. Therefore all worship of God is conducive to salvation, and consequently none is pernicious.
Objection 2: Further, it is the same God that is worshiped by the just in any age of the world. Now before the giving of the Law the just worshiped God in whatever manner they pleased, without committing mortal sin: wherefore Jacob bound himself by his own vow to a special kind of worship, as related in Genesis 28. Therefore now also no worship of God is pernicious.
Objection 3: Further, nothing pernicious is tolerated in the Church. Yet the Church tolerates various rites of divine worship: wherefore Gregory, replying to Augustine, bishop of the English (Regist. xi, ep. 64), who stated that there existed in the churches various customs in the celebration of Mass, wrote: "I wish you to choose carefully whatever you find likely to be most pleasing to God, whether in the Roman territory, or in the land of the Gauls, or in any part of the Church." Therefore no way of worshiping God is pernicious.
On the contrary, Augustine [*Jerome (Ep. lxxv, ad Aug.) See Opp. August. Ep. lxxxii] in a letter to Jerome (and the words are quoted in a gloss on Gal. 2:14) says that "after the Gospel truth had been preached the legal observances became deadly," and yet these observances belonged to the worship of God. Therefore there can be something deadly in the divine worship.
I answer that, As Augustine states (Cont. Mendac. xiv), "a most pernicious lie is that which is uttered in matters pertaining to Christian religion." Now it is a lie if one signify outwardly that which is contrary to the truth. But just as a thing is signified by word, so it is by deed: and it is in this signification by deed that the outward worship of religion consists, as shown above (Q[81], A[7]). Consequently, if anything false is signified by outward worship, this worship will be pernicious.
Now this happens in two ways. In the first place, it happens on the part of the thing signified, through the worship signifying something discordant therefrom: and in this way, at the time of the New Law, the mysteries of Christ being already accomplished, it is pernicious to make use of the ceremonies of the Old Law whereby the mysteries of Christ were foreshadowed as things to come: just as it would be pernicious for anyone to declare that Christ has yet to suffer. In the second place, falsehood in outward worship occurs on the part of the worshiper, and especially in common worship which is offered by ministers impersonating the whole Church. For even as he would be guilty of falsehood who would, in the name of another person, proffer things that are not committed to him, so too does a man incur the guilt of falsehood who, on the part of the Church, gives worship to God contrary to the manner established by the Church or divine authority, and according to ecclesiastical custom. Hence Ambrose [*Comment. in 1 ad1 Cor. 11:27, quoted in the gloss of Peter Lombard] says: "He is unworthy who celebrates the mystery otherwise than Christ delivered it." For this reason, too, a gloss on Col. 2:23 says that superstition is "the use of human observances under the name of religion."
Reply to Objection 1: Since God is truth, to invoke God is to worship Him in spirit and truth, according to Jn. 4:23. Hence a worship that contains falsehood, is inconsistent with a salutary calling upon God.
Reply to Objection 2: Before the time of the Law the just were instructed by an inward instinct as to the way of worshiping God, and others followed them. But afterwards men were instructed by outward precepts about this matter, and it is wicked to disobey them.
Reply to Objection 3: The various customs of the Church in the divine worship are in no way contrary to the truth: wherefore we must observe them, and to disregard them is unlawful.
Objection 1: It would seem that there cannot be excess in the worship of God. It is written (Ecclus. 43:32): "Glorify the Lord as much as ever you can, for He will yet far exceed." Now the divine worship is directed to the glorification of God. Therefore there can be no excess in it.
Objection 2: Further, outward worship is a profession of inward worship, "whereby God is worshiped with faith, hope, and charity," as Augustine says (Enchiridion iii). Now there can be no excess in faith, hope, and charity. Neither, therefore, can there be in the worship of God.
Objection 3: Further, to worship God consists in offering to Him what we have received from Him. But we have received all our goods from God. Therefore if we do all that we possibly can for God's honor, there will be no excess in the divine worship.
On the contrary, Augustine says (De Doctr. Christ. ii, 18) "that the good and true Christian rejects also superstitious fancies, from Holy Writ." But Holy Writ teaches us to worship God. Therefore there can be superstition by reason of excess even in the worship of God.
I answer that, A thing is said to be in excess in two ways. First, with regard to absolute quantity, and in this way there cannot be excess in the worship of God, because whatever man does is less than he owes God. Secondly, a thing is in excess with regard to quantity of proportion, through not being proportionate to its end. Now the end of divine worship is that man may give glory to God, and submit to Him in mind and body. Consequently, whatever a man may do conducing to God's glory, and subjecting his mind to God, and his body, too, by a moderate curbing of the concupiscences, is not excessive in the divine worship, provided it be in accordance with the commandments of God and of the Church, and in keeping with the customs of those among whom he lives.
On the other hand if that which is done be, in itself, not conducive to God's glory, nor raise man's mind to God, nor curb inordinate concupiscence, or again if it be not in accordance with the commandments of God and of the Church, or if it be contrary to the general custom---which, according to Augustine [*Ad Casulan. Ep. xxxvi], "has the force of law"---all this must be reckoned excessive and superstitious, because consisting, as it does, of mere externals, it has no connection with the internal worship of God. Hence Augustine (De Vera Relig. iii) quotes the words of Lk. 17:21, "The kingdom of God is within you," against the "superstitious," those, to wit, who pay more attention to externals.
Reply to Objection 1: The glorification of God implies that what is done is done for God's glory: and this excludes the excess denoted by superstition.
Reply to Objection 2: Faith, hope and charity subject the mind to God, so that there can be nothing excessive in them. It is different with external acts, which sometimes have no connection with these virtues.
Reply to Objection 3: This argument considers excess by way of absolute quantity.