Objection 1. It seems that words are not required for the signification of the sacraments. For Augustine says (Contra Faust. xix): "What else is a corporeal sacrament but a kind of visible word?" Wherefore to add words to the sensible things in the sacraments seems to be the same as to add words to words. But this is superfluous. Therefore words are not required besides the sensible things in the sacraments.
Objection 2. Further, a sacrament is some one thing, but it does not seem possible to make one thing of those that belong to different genera. Since, therefore, sensible things and words are of different genera, for sensible things are the product of nature, but words, of reason; it seems that in the sacraments, words are not required besides sensible things.
Objection 3. Further, the sacraments of the New Law succeed those of the Old Law: since "the former were instituted when the latter were abolished," as Augustine says (Contra Faust. xix). But no form of words was required in the sacraments of the Old Law. Therefore neither is it required in those of the New Law.
On the contrary, The Apostle says (Ephesians 5:25-26): "Christ loved the Church, and delivered Himself up for it; that He might sanctify it, cleansing it by the laver of water in the word of life." And Augustine says (Tract. xxx in Joan.): "The word is added to the element, and this becomes a sacrament."
I answer that, The sacraments, as stated above (2,3), are employed as signs for man's sanctification. Consequently they can be considered in three ways: and in each way it is fitting for words to be added to the sensible signs. For in the first place they can be considered in regard to the cause of sanctification, which is the Word incarnate: to Whom the sacraments have a certain conformity, in that the word is joined to the sensible sign, just as in the mystery of Incarnation the Word of God is united to sensible flesh.
Secondly, sacraments may be considered on the part of man who is sanctified, and who is composed of soul and body: to whom the sacramental remedy is adjusted, since it touches the body through the sensible element, and the soul through faith in the words. Hence Augustine says (Tract. lxxx in Joan.) on John 15:3, "Now you are clean by reason of the word," etc.: "Whence hath water this so great virtue, to touch the body and wash the heart, but by the word doing it, not because it is spoken, but because it is believed?"
Thirdly, a sacrament may be considered on the part of the sacramental signification. Now Augustine says (De Doctr. Christ. ii) that "words are the principal signs used by men"; because words can be formed in various ways for the purpose of signifying various mental concepts, so that we are able to express our thoughts with greater distinctness by means of words. And therefore in order to insure the perfection of sacramental signification it was necessary to determine the signification of the sensible things by means of certain words. For water may signify both a cleansing by reason of its humidity, and refreshment by reason of its being cool: but when we say, "I baptize thee," it is clear that we use water in baptism in order to signify a spiritual cleansing.
Reply to Objection 1. The sensible elements of the sacraments are called words by way of a certain likeness, in so far as they partake of a certain significative power, which resides principally in the very words, as stated above. Consequently it is not a superfluous repetition to add words to the visible element in the sacraments; because one determines the other, as stated above.
Reply to Objection 2. Although words and other sensible things are not in the same genus, considered in their natures, yet have they something in common as to the thing signified by them: which is more perfectly done in words than in other things. Wherefore in the sacraments, words and things, like form and matter, combine in the formation of one thing, in so far as the signification of things is completed by means of words, as above stated. And under words are comprised also sensible actions, such as cleansing and anointing and such like: because they have a like signification with the things.
Reply to Objection 3. As Augustine says (Contra Faust. xix), the sacraments of things present should be different from sacraments of things to come. Now the sacraments of the Old Law foretold the coming of Christ. Consequently they did not signify Christ so clearly as the sacraments of the New Law, which flow from Christ Himself, and have a certain likeness to Him, as stated above. Nevertheless in the Old Law, certain words were used in things pertaining to the worship of God, both by the priests, who were the ministers of those sacraments, according to Numbers 6:23-24: "Thus shall you bless the children of Israel, and you shall say to them: The Lord bless thee," etc.; and by those who made use of those sacraments, according to Deuteronomy 26:3: "I profess this day before the Lord thy God," etc.
Source: Summa Theologica (New Advent)