At the heart of The Lord's preaching, while he walked among us, was the call to repentance from sin.  Peter of Blois (d. 1211), a Frenchman who acted in a diplomatic capacity in places as varied as southern Italy and England, wrote an essay on the Sacrament of Confession.  He writes:

"The Lord spoke to Moses, saying: 'A man on whose skin a different color appears shall go to the priest and show himself to him' (Leviticus 13, 1).  This means that the confession of sin -- the showing of the spot -- must be made to a priest.  Augustine says: 'Let no one say to himself, I shall confess my sin secretly, and I shall do penance before God.'  If this confession were sufficient, the keys of the kingdom were given to Peter in vain.  It would be in vain that it was told him, 'Whatever you loose on earth will be loosed in heaven.'  All sins are completely washed away by confession.  And, just as 'it is believed in the heart unto justice, so confession of the mouth 'is made for salvation' (Romans 10, 10).  Great is the power of confession.  In his book, On Paradise, blessed Ambrose writes, 'Confession washes the soul, confession opens Paradise.'  A devout confession is like a powerful drink that carefully searches hearts and bowels, extending even to the division between the spirit and soul, emptying out noxious matter from the marrow of the soul. . . . Confessing is so efficacious that if a sinner discloses his guilt in confession, God covers it, excuses the one who is accusing himself, and forgives the one who is acknowledging his sin.  According to Solomon, 'He who hides his sins shall not prosper, but he who confesses and forsakes them shall receive mercy' (Proverbs 28, 13)."