"As the Father hath sent me, I also send you. When he had said this, he breathed on them; and he said to them: Receive ye the Holy Ghost. Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained." (John 20:21-23).
"Jesus’ call to conversion and penance... does not aim first at outward works... but at the conversion of the heart, interior conversion." (Catechism Section 1430)
"The human heart is heavy and hardened. God must give man a new heart. Conversion is first of all a work of the grace of God who makes our hearts return to him." (Catechism Section 1432)
"The grace conferred is deliverance from the guilt of sin and, in the case of mortal sin, from its eternal punishment; hence also reconciliation with God, justification." -Catechism
St. Cyril of Alexandria: "Men filled with the spirit of God forgive sins in two ways, either by admitting to baptism those who are worthy or by pardoning the penitent children of the Church." (in Joan., 1, 12 in P.G., LXXIV, 722)