Catechism of the Catholic Church §1850 — excerpt
Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight." Sin sets itself against God's love for us and turns our hearts away from it.
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.
Also cites: St. Augustine, De civ. Dei 14, 28: PL 41, 436