Objection 1. It seems that water is not the proper matter of Baptism. For Baptism, according to Dionysius (Eccl. Hier. v) and Damascene (De Fide Orth. iv), has a power of enlightening. But enlightenment is a special characteristic of fire. Therefore Baptism should be conferred with fire rather than with water: and all the more since John the Baptist said when foretelling Christ's Baptism (Matthew 3:11): "He shall baptize you in the Holy Ghost and fire."
Objection 2. Further, the washing away of sins is signified in Baptism. But many other things besides water are employed in washing, such as wine, oil, and such like. Therefore Baptism can be conferred with these also; and consequently water is not the proper matter of Baptism.
Objection 3. Further, the sacraments of the Church flowed from the side of Christ hanging on the cross, as stated above (Question 62, Article 5). But not only water flowed therefrom, but also blood. Therefore it seems that Baptism can also be conferred with blood. And this seems to be more in keeping with the effect of Baptism, because it is written (Apocalypse 1:5): "(Who) washed us from our sins in His own blood."
Objection 4. Further, as Augustine (cf. Master of the Sentences, iv, 3) and Bede (Exposit. in Luc. iii, 21) say, Christ, by "the touch of His most pure flesh, endowed the waters with a regenerating and cleansing virtue." But all waters are not connected with the waters of the Jordan which Christ touched with His flesh. Consequently it seems that Baptism cannot be conferred with any water; and therefore water, as such, is not the proper matter of Baptism.
Objection 5. Further, if water, as such, were the proper matter of Baptism, there would be no need to do anything to the water before using it for Baptism. But in solemn Baptism the water which is used for baptizing, is exorcized and blessed. Therefore it seems that water, as such, is not the proper matter of Baptism.
On the contrary, our Lord said (John 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, By Divine institution water is the proper matter of Baptism; and with reason. First, by reason of the very nature of Baptism, which is a regeneration unto spiritual life. And this answers to the nature of water in a special degree; wherefore seeds, from which all living things, viz. plants and animals are generated, are moist and akin to water. For this reason certain philosophers held that water is the first principle of all things.
Secondly, in regard to the effects of Baptism, to which the properties of water correspond. For by reason of its moistness it cleanses; and hence it fittingly signifies and causes the cleansing from sins. By reason of its coolness it tempers superfluous heat: wherefore it fittingly mitigates the concupiscence of the fomes. By reason of its transparency, it is susceptive of light; hence its adaptability to Baptism as the "sacrament of Faith."
Thirdly, because it is suitable for the signification of the mysteries of Christ, by which we are justified. For, as Chrysostom says (Hom. xxv in Joan.) on John 3:5, "Unless a man be born again," etc., "When we dip our heads under the water as in a kind of tomb our old man is buried, and being submerged is hidden below, and thence he rises again renewed."
Fourthly, because by being so universal and abundant, it is a matter suitable to our need of this sacrament: for it can easily be obtained everywhere.
Reply to Objection 1. Fire enlightens actively. But he who is baptized does not become an enlightener, but is enlightened by faith, which "cometh by hearing" (Romans 10:17). Consequently water is more suitable, than fire, for Baptism.
But when we find it said: "He shall baptize you in the Holy Ghost and fire," we may understand fire, as Jerome says (In Matth. ii), to mean the Holy Ghost, Who appeared above the disciples under the form of fiery tongues (Acts 2:3). Or we may understand it to mean tribulation, as Chrysostom says (Hom. iii in Matth.): because tribulation washes away sin, and tempers concupiscence. Or again, as Hilary says (Super Matth. ii) that "when we have been baptized in the Holy Ghost," we still have to be "perfected by the fire of the judgment."
Reply to Objection 2. Wine and oil are not so commonly used for washing, as water. Neither do they wash so efficiently: for whatever is washed with them, contracts a certain smell therefrom; which is not the case if water be used. Moreover, they are not so universal or so abundant as water.
Reply to Objection 3. Water flowed from Christ's side to wash us; blood, to redeem us. Wherefore blood belongs to the sacrament of the Eucharist, while water belongs to the sacrament of Baptism. Yet this latter sacrament derives its cleansing virtue from the power of Christ's blood.
Reply to Objection 4. Christ's power flowed into all waters, by reason of, not connection of place, but likeness of species, as Augustine says in a sermon on the Epiphany (Append. Serm. cxxxv): "The blessing that flowed from the Saviour's Baptism, like a mystic river, swelled the course of every stream, and filled the channels of every spring."
Reply to Objection 5. The blessing of the water is not essential to Baptism, but belongs to a certain solemnity, whereby the devotion of the faithful is aroused, and the cunning of the devil hindered from impeding the baptismal effect.
Source: Summa Theologica (New Advent)