This sacrament of Reconciliation, as it is called, is firmly grounded both in Scripture and in early, constant Christian Tradition. The priestly authority to represent God as an ambassador of his mercy was granted by Jesus to St. Peter and the other apostles—and by extension, to the priests they and their successors ordained: “Whatever you bind [that is, by imposing penance] on earth shall be bound in heaven; and whatever you loose [that is, forgive in God’s name] on earth shall be loosed in heaven” (Mt 16:19; 18:18). “Whose sins you forgive are forgiven them, and whose sins you retain are retained” (Jn 20:23).
In the early Church, confession of grave sin was often made to the whole Christian assembly as well as the priests. Certain instructions in the Letter of James are suggestive of this early practice. When sick, believers were to call the presbyters (priests). The priests were then to anoint them with oil and pray for them. In confessing their sins, the sick could be healed and forgiven (see Jas 5:13-16).
Catholics are obligated to repent of all mortal, or grave, sins (contrition) and to confess them to a priest in order to be absolved. The penitent performs the assigned penance to repair the harm caused by sin and to reestablish habits that lead to holiness. The absolution imparted by the priest is not a mere expression of hope, but a sacramental, objective reality.