CCC 1460 The penance the confessor imposes must take into account the penitent’s personal situation and must seek his spiritual good. It must correspond as far as possible with the gravity and nature of the sins committed. It can consist of prayer, an offering, works of mercy, service of neighbor, voluntary self-denial, sacrifices, and above all the patient acceptance of the cross we must bear. Such penances help configure us to Christ, who alone expiated our sins once for all. They allow us to become co-heirs with the risen Christ, “provided we suffer with him.”¹
The satisfaction that we make for our sins, however, is not so much ours as though it were not done through Jesus Christ. We who can do nothing ourselves, as if just by ourselves, can do all things with the cooperation of “him who strengthens” us. Thus man has nothing of which to boast, but all our boasting is in Christ. .. in whom we make satisfaction by bringing forth “fruits that befit repentance.” These fruits have their efficacy from him, by him they are offered to the Father, and through him they are accepted by the Father.²

1 Rom 8:17; Rom 3:25; 1 Jn 2:1-2; cf. Council of Trent (1551): DS 1690.
2 Council of Trent (1551): DS 1691; cf. Phil 4:13; 1 Cor 1:31; 2 Cor 10:17; Gal 6:14; Lk 3:8.