After his Resurrection, Christ sent his apostles "so that repentance and forgiveness of sins should be preached in his name to all nations." The apostles and their successors carry out this "ministry of reconciliation," not only by announcing to men God's forgiveness merited for us by Christ, and calling them to conversion and faith; but also by communicating to them the forgiveness of sins in Baptism, and reconciling them with God and with the Church through the power of the keys, received from Christ:
[The Church] has received the keys of the Kingdom of heaven so that, in her, sins may be forgiven through Christ's blood and the Holy Spirit's action. In this Church, the soul dead through sin comes back to life in order to live with Christ, whose grace has saved us.
Source: Catechism of the Catholic Church (Vatican.va)