Objection 1. It seems that not all sins are taken away by Baptism. For Baptism is a spiritual regeneration, which corresponds to carnal generation. But by carnal generation man contracts none but original sin. Therefore none but original sin is taken away by Baptism.
Objection 2. Further, Penance is a sufficient cause of the remission of actual sins. But penance is required in adults before Baptism, according to Acts 2:38: "Do penance and be baptized every one of you." Therefore Baptism has nothing to do with the remission of actual sins.
Objection 3. Further, various diseases demand various remedies: because as Jerome says on Mark 9:27-28: "What is a cure for the heel is no cure for the eye." But original sin, which is taken away by Baptism, is generically distinct from actual sin. Therefore not all sins are taken away by Baptism.
On the contrary, 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.
Reply to Objection 1. As the Apostle says (Romans 5:15-16), the sin of Adam was not so far-reaching as the gift of Christ, which is bestowed in Baptism: "for judgment was by one unto condemnation; but grace is of many offenses, unto justification." Wherefore Augustine says in his book on Infant Baptism (De Pecc. Merit. et Remiss. i), that "in carnal generation, original sin alone is contracted; but when we are born again of the Spirit, not only original sin but also wilful sin is forgiven."
Reply to Objection 2. No sin can be forgiven save by the power of Christ's Passion: hence the Apostle says (Hebrews 9:22) that "without shedding of blood there is no remission." Consequently no movement of the human will suffices for the remission of sin, unless there be faith in Christ's Passion, and the purpose of participating in it, either by receiving Baptism, or by submitting to the keys of the Church. Therefore when an adult approaches Baptism, he does indeed receive the forgiveness of all his sins through his purpose of being baptized, but more perfectly through the actual reception of Baptism.
Reply to Objection 3. This argument is true of special remedies. But Baptism operates by the power of Christ's Passion, which is the universal remedy for all sins; and so by Baptism all sins are loosed.
Source: Summa Theologica (New Advent)