Objection 1. It seems that works of satisfaction should be enjoined on sinners that have been baptized. For God's justice seems to demand that a man should be punished for every sin of his, according to Ecclesiastes 12:14: "All things that are done, God will bring into judgment." But works of satisfaction are enjoined on sinners in punishment of past sins. Therefore it seems that works of satisfaction should be enjoined on sinners that have been baptized.
Objection 2. Further, by means of works of satisfaction sinners recently converted are drilled into righteousness, and are made to avoid the occasions of sin: "for satisfaction consists in extirpating the causes of vice, and closing the doors to sin" (De Eccl. Dogm. iv). But this is most necessary in the case of those who have been baptized recently. Therefore it seems that works of satisfaction should be enjoined on sinners.
Objection 3. Further, man owes satisfaction to God not less than to his neighbor. But if those who were recently baptized have injured their neighbor, they should be told to make reparation to God by works of penance.
On the contrary, Ambrose commenting on Romans 11:29: "The gifts and the calling of God are without repentance," says: "The grace of God requires neither sighs nor groans in Baptism, nor indeed any work at all, but faith alone; and remits all, gratis."
I answer that, As the Apostle says (Romans 6:3-4), "all we who are baptized in Christ Jesus, are baptized in His death: for we are buried together with Him, by Baptism unto death"; which is to say that by Baptism man is incorporated in the very death of Christ. Now it is manifest from what has been said above (48, 2,4; 49, 3) that Christ's death satisfied sufficiently for sins, "not for ours only, but also for those of the whole world," according to 1 John 2:2. Consequently no kind of satisfaction should be enjoined on one who is being baptized, for any sins whatever: and this would be to dishonor the Passion and death of Christ, as being insufficient for the plenary satisfaction for the sins of those who were to be baptized.
Reply to Objection 1. As Augustine says in his book on Infant Baptism (De Pecc. Merit. et Remiss. i), "the effect of Baptism is to make those, who are baptized, to be incorporated in Christ as His members." Wherefore the very pains of Christ were satisfactory for the sins of those who were to be baptized; just as the pain of one member can be satisfactory for the sin of another member. Hence it is written (Isaiah 53:4): "Surely He hath borne our infirmities and carried our sorrows."
Reply to Objection 2. Those who have been lately baptized should be drilled into righteousness, not by penal, but by "easy works, so as to advance to perfection by taking exercise, as infants by taking milk," as a gloss says on Psalm 130:2: "As a child that is weaned is towards his mother." For this reason did our Lord excuse His disciples from fasting when they were recently converted, as we read in Matthew 9:14-15: and the same is written 1 Peter 2:2: "As new-born babes desire... milk... that thereby you may grow unto salvation."
Reply to Objection 3. To restore what has been ill taken from one's neighbor, and to make satisfaction for wrong done to him, is to cease from sin: for the very fact of retaining what belongs to another and of not being reconciled to one's neighbor, is a sin. Wherefore those who are baptized should be enjoined to make satisfaction to their neighbor, as also to desist from sin. But they are not to be enjoined to suffer any punishment for past sins.
Source: Summa Theologica (New Advent)