Catechism of the Catholic Church §2485 — excerpt
By its very nature, lying is to be condemned. It is a profanation of speech, whereas the purpose of speech is to communicate known truth to others.
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.