1265–1273 · Thomas Aquinas, Summa Theologica III, q. 68, a. 9, co.
I answer that, As the Apostle says (Rom. 5:17), "if by one man's offense death reigned through one," namely Adam, "much more they who receive abundance of grace, and of the gift, and of justice, shall reign in life through one, Jesus Christ." Now children contract original sin from the sin of Adam; which is made clear by the fact that they are under the ban of death, which "passed upon all" on account of the sin of the first man, as the Apostle says in the same passage (Rom. 5:12). Much more, therefore, can children receive grace through Christ, so as to reign in eternal life. But our Lord Himself said (Jn. 3:5): "Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God." Consequently it became necessary to baptize children, that, as in birth they incurred damnation through Adam so in a second birth they might obtain salvation through Christ. Moreover it was fitting that children should receive Baptism, in order that being reared from childhood in things pertaining to the Christian mode of life, they may the more easily persevere therein; according to Prov. 22:5: "A young man according to his way, even when he is old, he will not depart from it."