20 But among all these powers of the priest over the Mystical Body of Christ for the benefit of the faithful, there is one of which the simple mention made above will not content Us. This is that power which, as St. John Chrysostom says: "God gave neither to Angels nor Archangels" - the power to remit sins. "Whose sins you shall forgive they are forgiven them: and whose sins you shall retain they are retained"; a tremendous power, so peculiar to God that even human pride could not make the mind conceive that it could be given to man. "Who can forgive sins but God alone?" And, when we see it exercised by a mere man there is reason to ask ourselves, not, indeed, with pharisaical scandal, but with reverent surprise at such a dignity: "Who is this that forgiveth sins also?" But it is so: the God-Man who possessed the "power on earth to forgive sins" willed to hand it on to His priests; to relieve, in His divine generosity and mercy, the need of moral purification which is rooted in the human heart.
Source: Ad Catholici Sacerdotii (Vatican.va)