Catechism of the Catholic Church §1874 — excerpt
To choose deliberately - that is, both knowing it and willing it - something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin.
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.