Thomas Aquinas

“ Then is a sign ambiguous and the occasion of deception, when it signifies many things not ordained to one another. But when it signifies many things inasmuch as, through being mutually ordained, they form one thing, then the sign is not ambiguous but certain: thus this word "man" signifies the soul and body inasmuch as together they form the human nature. In this way a sacrament signifies the three things aforesaid, inasmuch as by being in a certain order they are one thing. ”

“ Now it is part of man's nature to acquire knowledge of the intelligible from the sensible. But a sign is that by means of which one attains to the knowledge of something else. Consequently, since the sacred things which are signified by the sacraments, are the spiritual and intelligible goods by means of which man is sanctified, it follows that the sacramental signs consist in sensible things: just as in the Divine Scriptures spiritual things are set before us under the guise of things sensible. ”

“ 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. ”

“ Now in all things composed of matter and form, the determining principle is on the part of the form, which is as it were the end and terminus of the matter. Consequently for the being of a thing the need of a determinate form is prior to the need of determinate matter: for determinate matter is needed that it may be adapted to the determinate form. Since, therefore, in the sacraments determinate sensible things are required, which are as the sacramental matter, much more is there need in them of a determinate form of words. ”

“ Therefore it seems that man should not have been restricted, especially under the New Law, to the use of any determinate thing in the sacraments.
On the contrary, our Lord said (Jn. 3:5) : "Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God."
I answer that, In the use of the sacraments two things may be considered, namely, the worship of God, and the sanctification of man: the former of which pertains to man as referred to God, and the latter pertains to God in reference to man. ”

“ So also among sensible things, that one is used for the sacramental signification which is most commonly employed for the action by which the sacramental effect is signified: thus water is most commonly used by men for bodily cleansing, by which the spiritual cleansing is signified: and therefore water is employed as the matter of baptism.
Reply to Objection 3: If he who corrupts the pronunciation of the sacramental words---does so on purpose, he does not seem to intend to do what the Church intends: and thus the sacrament seems to be defective. ”

“ Consequently, just as under the state of the Law of nature man was moved by inward instinct and without any outward law, to worship God, so also the sensible things to be employed in the worship of God were determined by inward instinct. ”

“ But later on it became necessary for a law to be given (to man) from without: both because the Law of nature had become obscured by man's sins; and in order to signify more expressly the grace of Christ, by which the human race is sanctified. ”