11 I have written this, not in order to confound you, but that the examples of these kings may stir you up to put away this sin from your kingdom, for you will do it away by humbling your soul before God. You are a man, and it has come upon you, conquer it. Sin is not done away but by tears and penitence. Neither angel can do it, nor archangel. The Lord Himself, Who alone can say, “I am with you,” if we have sinned, does not forgive any but those who repent.
Source: Letters (New Advent)