VI. 1440. Sin is before all else an offence 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.
Only God forgives sin
1441. Only God forgives sins. (Mk 2:7) Since he is the Son of God, Jesus says of himself, “The Son of man has authority on earth to forgive sins” and exercises this divine power: “Your sins are forgiven.” (Mk 2:5, 10; Lk 7:48) Further, by virtue of his divine authority he gives this power to men to exercise in his name. (cf. Jn 20:21-23)
1442. Christ has willed that in her prayer and life and action his whole Church should be a sign and instrument of the forgiveness and reconciliation that he acquired for us at the price of his blood. But he entrusted the exercise of the power of absolution to the apostolic ministry which he charged with the “ministry of reconciliation.” (2Cor 5:18) The apostle is sent out “on behalf of Christ” with “God making his appeal” through him and pleading: “Be reconciled to God.” (2Cor 5:20)
Reconciliation with the Church
1443. During his public life Jesus not only forgave sins, but also made plain the effect of this forgiveness: he reintegrated forgiven sinners into the community of the People of God from which sin had alienated or even excluded them. A remarkable sign of this is the fact that Jesus receives sinners at his table, a gesture that expresses in an astonishing way both God’s forgiveness and the return to the bosom of the People of God. (cf. Lk15; 19:9)
1444. In imparting to his apostles his own power to forgive sins the Lord also gives them the authority to reconcile sinners with the Church. The ecclesial dimension of their task is expressed most notably in Christ’s solemn words to Simon Peter: “I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.” (Mt16:19; cf. Mt 18:18; 28:16-20) The office of binding and loosing which was given to Peter was also assigned to the college of the apostles united to its head. (LG 22 para 2)
1445. The words bind and loose mean: whomever you exclude from your communion, will be excluded from communion with God; whomever you receive anew into your communion, God will welcome back into his. Reconciliation with the Church is inseparable from reconciliation with God.
The sacrament of forgiveness
1446. Christ instituted the sacrament of Penance for all sinful members of his Church: above all for those who, since Baptism, have fallen into grave sin, and have thus lost their baptismal grace and wounded ecclesial communion. It is to them that the sacrament of Penance offers a new possibility to convert and to recover the grace of justification. The Fathers of the Church present this sacrament as “the second plank [of salvation] after the shipwreck which is the loss of grace.” (Tertullian, De Pænit. 4, 2: PL 1, 1343; cf. Council of Trent (1547): DS 1542.
– From: The Catechism of the Catholic Church