Objection 1. It seems that Baptism is not the mere washing. For the washing of the body is something transitory: but Baptism is something permanent. Therefore Baptism is not the mere washing; but rather is it "the regeneration, the seal, the safeguarding, the enlightenment," as Damascene says (De Fide Orth. iv).
Objection 2. Further, Hugh of St. Victor says (De Sacram. ii) that "Baptism is water sanctified by God's word for the blotting out of sins." But the washing itself is not water, but a certain use of water.
Objection 3. Further, Augustine says (Tract. lxxx super Joan.): "The word is added to the element, and this becomes a sacrament." Now, the element is the water. Therefore Baptism is the water and not the washing.
On the contrary, It is written (Sirach 34:30): "He that washeth himself [baptizatur] after touching the dead, if he touch him again, what does his washing avail?" It seems, therefore, that Baptism is the washing or bathing.
I answer that, In the sacrament of Baptism, three things may be considered: namely, that which is "sacrament only"; that which is "reality and sacrament"; and that which is "reality only." That which is sacrament only, is something visible and outward; the sign, namely, of the inward effect: for such is the very nature of a sacrament. And this outward something that can be perceived by the sense is both the water itself and its use, which is the washing. Hence some have thought that the water itself is the sacrament: which seems to be the meaning of the passage quoted from Hugh of St. Victor. For in the general definition of a sacrament he says that it is "a material element": and in defining Baptism he says it is "water."
But this is not true. For since the sacraments of the New Law effect a certain sanctification, there the sacrament is completed where the sanctification is completed. Now, the sanctification is not completed in water; but a certain sanctifying instrumental virtue, not permanent but transient, passes from the water, in which it is, into man who is the subject of true sanctification. Consequently the sacrament is not completed in the very water, but in applying the water to man, i.e. in the washing. Hence the Master (iv, 3) says that "Baptism is the outward washing of the body done together with the prescribed form of words."
The Baptismal character is both reality and sacrament: because it is something real signified by the outward washing; and a sacramental sign of the inward justification: and this last is the reality only, in this sacrament--namely, the reality signified and not signifying.
Reply to Objection 1. That which is both sacrament and reality--i.e. the character--and that which is reality only--i.e. the inward justification--remain: the character remains and is indelible, as stated above (Question 63, Article 5); the justification remains, but can be lost. Consequently Damascene defined Baptism, not as to that which is done outwardly, and is the sacrament only; but as to that which is inward. Hence he sets down two things as pertaining to the character--namely, "seal" and "safeguarding"; inasmuch as the character which is called a seal, so far as itself is concerned, safeguards the soul in good. He also sets down two things as pertaining to the ultimate reality of the sacrament--namely, "regeneration" which refers to the fact that man by being baptized begins the new life of righteousness; and "enlightenment," which refers especially to faith, by which man receives spiritual life, according to Habakkuk 2 (Hebrews 10:38; cf. Habakkuk 2:4): "But (My) just man liveth by faith"; and Baptism is a sort of protestation of faith; whence it is called the "Sacrament of Faith." Likewise Dionysius defined Baptism by its relation to the other sacraments, saying (Eccl. Hier. ii) that it is "the principle that forms the habits of the soul for the reception of those most holy words and sacraments"; and again by its relation to heavenly glory, which is the universal end of all the sacraments, when he adds, "preparing the way for us, whereby we mount to the repose of the heavenly kingdom"; and again as to the beginning of spiritual life, when he adds, "the conferring of our most sacred and Godlike regeneration."
Reply to Objection 2. As already stated, the opinion of Hugh of St. Victor on this question is not to be followed. Nevertheless the saying that "Baptism is water" may be verified in so far as water is the material principle of Baptism: and thus there would be "causal predication."
Reply to Objection 3. When the words are added, the element becomes a sacrament, not in the element itself, but in man, to whom the element is applied, by being used in washing him. Indeed, this is signified by those very words which are added to the element, when we say: "I baptize thee," etc.
Source: Summa Theologica (New Advent)