Roman Catechism 12.11

Chapter 12 — Part I: The Creed — Article X: The Forgiveness of Sins

Section 11. With great truth, therefore, have the ancient Fathers declared that God alone can forgive sins, and that to His infinite goodness and power alone is so wonderful a work to be referred. I am he, says the Lord Himself, by the mouth of His Prophet, I am he who blotteth out your iniquities.