Catechism of the Catholic Church §217 — excerpt
God is also truthful when he reveals himself - the teaching that comes from God is "true instruction". When he sends his Son into the world it will be "to bear witness to the truth": "We know that the Son of God has come and has given us understanding, to know him who is true."
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.