1. According to Catholic teaching on the sacrament of penance, even forgiven sin deserves some punishment (see DS 1689/904). Being sorry and being forgiven are not enough; something must be done to undo sin’s existential consequences. Trent teaches that it belongs to God’s mercy not to waive all punishment when he forgives sins. For in making satisfaction, a sinner learns the seriousness of sin, is deterred from future sin, healed of the self-mutilation of sin by acts of virtue, and overcomes the bad habits acquired in living sinfully (see DS 1690/904).
2. Thus the punishment required for forgiven sin should not be thought of as an evil which God imposes to get even with the sinner. It is, rather, a way of repairing the damages caused by wrongdoing. Mortal sin is a betrayal of the Church and a sort of adultery against God. Besides being sorry and being restored to communion, one must work to regain one’s spiritual health and make up for the damage to the relationship (see S.t., 3, q. 86, a. 4; sup., q. 12, a. 3).
The situation is like that of a man who has committed adultery, repented, and been forgiven. It still is necessary that by more generous love he give his wife more of himself to make up to her for that of himself which he wrongfully took from her (see S.t., sup., q. 15, a. 1). The repentant adulterous husband must try to compensate in happiness for the misery he has caused. So we with God. Although God is neither hurt nor pleased by us in a merely human way, still our relationship does matter to him, and when we are unfaithful, we need to make amends.
3. In making reparation or satisfaction for our sins we are made like Jesus, who satisfied for our sins (see S.t., 3, q. 49, a. 1; q. 62, a. 5; sup., q. 13). The reparation we make is not independent, isolated; it is made through him, for of ourselves we can do nothing (see DS 1689–91/904). But through Jesus, our satisfaction for sin is a real contribution to our own sanctification. In making this contribution, the sinner, degraded by sin, is ennobled. Thus the grace of God is the greater in not only forgiving the sin but ennobling the sinner into the likeness of Jesus.
4. In sum, satisfaction for sin belongs to the sacrament of penance, and a “penance” is imposed for this purpose (see DS 1692/805). This, however, is only a token of the will to make reparation; above and beyond this token, all the repentant sinner suffers in life and accepts from God can serve as penance (see DS 1693/806; S.t., sup., q. 15, a. 2). The confessor may appropriately conclude the sacramental rite by praying that the passion of Christ, the intercession of Mary, and “whatever good you do and suffering you endure, heal your sins, help you to grow in holiness, and reward you with eternal life.”9
9. The Rites, 363.