This sacrament, often called Confession, is the sacrament by which, through the ministry of a priest, we are reconciled to God and to one another and our friendship with each of them is renewed, strengthened and sustained.
For the forgiveness of grave sin it is necessary that we should be sorry for our sin, wish to be forgiven and we intend, by Gods grace, not to sin again.
This is a very common and natural question as is the other question, why must a priest be involved? The answer to these questions lie in the fact that as an ordinary human thing, forgiveness, like worship, has to do with our life together, with other people.
Because by ordination the bishop or priest is authorised to represent the whole people of God at this moment of reconcilliation, to speak in the name of the whole church and thus, in the name of Christ himself, to accept us and to proclaim to us and assure us of our forgiveness and new life.
"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 labours for their conversion." It is called the sacrament of confession, since the disclosure or confession of sins to a priest is an essential element of this sacrament. In a profound sense it is also a "confession" - acknowledgment and praise - of the holiness of God and of his mercy toward sinful man.
It is called the sacrament of forgiveness, since by the priest's sacramental absolution God grants the penitent "pardon and peace."
It is called the sacrament of Reconciliation, because it imparts to the sinner the life of God who reconciles: "Be reconciled to God." He who lives by God's merciful love is ready to respond to the Lord's call: "Go; first be reconciled to your brother."
'Catechism of the Catholic Church'