It is written (Ezekiel 36:25): “I will pour upon you clean water, and you shall be cleansed from all your filthiness.”
I answer that, As the Apostle says (Romans 6:3), “all we, who are baptized in Christ Jesus, are baptized in His death.” And further on he concludes (Romans 6:11): “So do you also reckon that you are dead to sin, but alive unto God in Christ Jesus our Lord.” Hence it is clear that by Baptism man dies unto the oldness of sin, and begins to live unto the newness of grace. But every sin belongs to the primitive oldness. Consequently every sin is taken away by Baptism.
the Passion of Christ is communicated to every baptized person, so that he is healed just as if he himself had suffered and died. Now Christ’s Passion . . . is a sufficient satisfaction for all the sins of all men. Consequently he who is baptized, is freed from the debt of all punishment due to him for his sins, just as if he himself had offered sufficient satisfaction for all his sins.
As Augustine says in the book on Infant Baptism (De Pecc. Merit. et Remiss. i) “the effect of Baptism is that the baptized are incorporated in Christ as His members.” Now the fulness of grace and virtues flows from Christ theHead to all His members, according to John 1:16: “Of His fulness we all have received.” Hence it is clear that man receives grace and virtues in Baptism.
ST III, Q. 69, A. 1; 2; 4.
Learn more by reading my article on baptism – http://mattfradd.com/did-st-paul-downplay-baptism/