Objection 1. It seems that man is not freed by Baptism from all debt of punishment due to sin. For the Apostle says (Romans 13:1): "Those things that are of God are well ordered [Vulgate: 'Those that are, are ordained of God']." But guilt is not set in order save by punishment, as Augustine says (Ep. cxl). Therefore Baptism does not take away the debt of punishment due to sins already committed.
Objection 2. Further, the effect of a sacrament has a certain likeness to the sacrament itself; since the sacraments of the New Law "effect what they signify," as stated above (62, 1, ad 1). But the washing of Baptism has indeed a certain likeness with the cleansing from the stain of sin, but none, seemingly, with the remission of the debt of punishment. Therefore the debt of punishment is not taken away by Baptism.
Objection 3. Further, when the debt of punishment has been remitted, a man no longer deserves to be punished, and so it would be unjust to punish him. If, therefore, the debt of punishment be remitted by Baptism, it would be unjust, after Baptism, to hang a thief who had committed murder before. Consequently the severity of human legislation would be relaxed on account of Baptism; which is undesirable. Therefore Baptism does not remit the debt of punishment.
On the contrary, Ambrose, commenting on Romans 11:29, "The gifts and the calling of God ate without repentance," says: "The grace of God in Baptism remits all, gratis."
I answer that, As stated above (49, 3, ad 2; 68, 1,4,5) by Baptism a man is incorporated in the Passion and death of Christ, according to Romans 6:8: "If we be dead with Christ, we believe that we shall live also together with Christ." Hence it is clear that 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, as stated above (Question 68, Article 5), 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.
Reply to Objection 1. Since the pains of Christ's Passion are communicated to the person baptized, inasmuch as he is made a member of Christ, just as if he himself had borne those pains, his sins are set in order by the pains of Christ's Passion.
Reply to Objection 2. Water not only cleanses but also refreshes. And thus by refreshing it signifies the remission of the debt of punishment, just as by cleansing it signifies the washing away of guilt.
Reply to Objection 3. In punishments inflicted by a human tribunal, we have to consider not only what punishment a man deserves in respect of God, but also to what extent he is indebted to men who are hurt and scandalized by another's sin. Consequently, although a murderer is freed by Baptism from his debt of punishment in respect of God, he remains, nevertheless, in debt to men; and it is right that they should be edified at his punishment, since they were scandalized at his sin. But the sovereign may remit the penalty to such like out of kindness.
Source: Summa Theologica (New Advent)