Catechism of the Catholic Church §2163 — excerpt
False oaths call on God to be witness to a lie. Perjury is a grave offence against the Lord who is always faithful to his promises.
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.