Jesus' call to conversion and penance does not aim at outward works, but at the conversion of the heart, interior conversion. Interior repentance is a radical reorientation of the whole of life, a return, a conversion to God with all our heart, an end to sin, a turning away from evil, with a repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one's life, with hope in God's mercy and trust in the help of his grace.
This sacrament requires contrition on the part of the penitent. Contrition is 'sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again'. The penitent confesses his sins to the priest. If his sins have caused harm to others he must do whatever is possible to repair that harm (e.g. return stolen goods, restore the reputation of someone slandered, pay compensation for injuries). The confessor imposes a penance and reconciles the penitent with these words:
"God, the Father of mercies, through the death and resurrection of his Son has reconciled the world to himself and sent the Holy Spirit among us for the forgiveness of sins; through the ministry of the Church may God give you pardon and peace, and I absolve you from your sins in the name of the Father and of the Son and of the Holy Spirit. Amen."
This sacrament is a most important source of grace for us, not only when we have committed mortal sin, but for the ongoing growth of the Christian life in holiness. Although sin is forgiven by God in this sacrament, we may still need his healing grace to heal the damage and hurts that have been caused through sin.