Catechism of the Catholic Church §1871 — excerpt
Sin is an utterance, a deed, or a desire contrary to the eternal law (St. Augustine, Faust 22: PL 42, 418). It is an offense against God.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
