CIC 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

CIC 2813 In the waters of Baptism, we have been “washed. .. sanctified. .. justified in the name of the Lord Jesus Christ and in the Spirit of our God.”3 Our Father calls us to holiness in the whole of our life, and since “he is the source of [our] life in Christ Jesus, who became for us wisdom from God, and. .. sanctification,”4 both his glory and our life depend on the hallowing of his name in us and by us. Such is the urgency of our first petition.
By whom is God hallowed, since he is the one who hallows? But since he said, “You shall be holy to me; for I the LORD am holy,” we seek and ask that we who were sanctified in Baptism may persevere in what we have begun to be. And we ask this daily, for we need sanctification daily, so that we who fail daily may cleanse away our sins by being sanctified continually. .. We pray that this sanctification may remain in us.5

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.
3 2 Cor 6:11.
4 1 Cor 1:30; cf. 1 Thess 4:7.
5 St. Cyprian De Dom. orat. 12: PL 4, 527A; Lev 20:26.