Objection 1. It would seem that there cannot be excess in the worship of God. It is written (Sirach 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 Luke 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.
Source: Summa Theologica (New Advent)