21 Again Jesus said, “Peace be with you! As the Father has sent me, I am sending you.” 22 And with that He breathed on them and said, “Receive the Holy Spirit. 23 If you forgive anyone’s sins, their sins are forgiven; if you do not forgive them, they are not forgiven.”
Q. 776. What is Confession?
A. Confession is the telling of our sins to a duly authorized priest, for the purpose of obtaining forgiveness.
Q. 785. Which are the chief qualities of a good Confession?
A. The chief qualities of a good Confession are three: it must be humble, sincere, and entire.
Q. 795. Is it a grievous offense willfully to conceal a mortal sin in Confession?
A. It is a grievous offense willfully to conceal a mortal sin in Confession, because we thereby tell a lie to the Holy Ghost, and make our Confession worthless.
Catechism of the Catholic Church
1430 - Jesus' call to conversion and penance, like that of the prophets before him, does not aim first at outward works, "sackcloth and ashes," fasting and mortification, but at the conversion of the heart, interior conversion. Without this, such penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance.
1431 - Interior repentance is a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with 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 conversion of heart is accompanied by a salutary pain and sadness which the Fathers called animi cruciatus (affliction of spirit) and compunctio cordis(repentance of heart).
Act of Contrition
O my God, I am heartily sorry for having offended Thee, and I detest all my sins, because I dread the loss of heaven, and the pains of hell; but most of all because they offend Thee, my God, Who are all good and deserving of all my love. I firmly resolve, with the help of Thy grace, to confess my sins, to do penance, and to amend my life. Amen.