Take courage, son; thy sins are forgiven thee.” Matthew 9:2; also "Peace be to you! As the Father has sent Me, I also send you... Receive the Holy Spirit; whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained." John 20:21-23
Penance is the sacrament by which sins committed after Baptism are forgiven through the absolution of the priest. (BC 379)
"Those who approach the sacrament of Penance obtain pardon from God's mercy for the offense committed against him, and are, at the same time, reconciled with the Church which they have wounded by their sins and which by charity, by example, and by prayer labors for their conversion."1 (CCC 1422)
Sin is before all else an offense against God, a rupture of communion with him. At the same time it damages communion with the Church. For this reason conversion entails both God’s forgiveness and reconciliation with the Church, which are expressed and accomplished liturgically by the sacrament of Penance and Reconciliation.2 (CCC 1440)
"The whole power of the sacrament of Penance consists in restoring us to God's grace and joining us with him in an intimate friendship."3 Reconciliation with God is thus the purpose and effect of this sacrament. For those who receive the sacrament of Penance with contrite heart and religious disposition, reconciliation "is usually followed by peace and serenity of conscience with strong spiritual consolation."4 Indeed the sacrament of Reconciliation with God brings about a true "spiritual resurrection," restoration of the dignity and blessings of the life of the children of God, of which the most precious is friendship with God.5 (CCC 1468)
This sacrament reconciles us with the Church. Sin damages or even breaks fraternal communion. The sacrament of Penance repairs or restores it. In this sense it does not simply heal the one restored to ecclesial communion, but has also a revitalizing effect on the life of the Church which suffered from the sin of one of her members.6 Re-established or strengthened in the communion of saints, the sinner is made stronger by the exchange of spiritual goods among all the living members of the Body of Christ, whether still on pilgrimage or already in the heavenly homeland:7
It must be recalled that . . . this reconciliation with God leads, as it were, to other reconciliations, which repair the other breaches caused by sin. The forgiven penitent is reconciled with himself in his inmost being, where he regains his innermost truth. He is reconciled with his brethren whom he has in some way offended and wounded. He is reconciled with the Church. He is reconciled with all creation.8 (1469)
In this sacrament, the sinner, placing himself before the merciful judgment of God, anticipates in a certain way the judgment to which he will be subjected at the end of his earthly life. For it is now, in this life, that we are offered the choice between life and death, and it is only by the road of conversion that we can enter the Kingdom, from which one is excluded by grave sin.9 In converting to Christ through penance and faith, the sinner passes from death to life and "does not come into judgment."10 (1470)
1 LG 11 § 2.
2 Cf. LG 11.
3 Roman Catechism, II,V,18.
9 Cf. 1 Cor 5:11; Gal 5:19-21; Rev 22:15.