Catechism of the Catholic Church §1063 — excerpt
In the book of the prophet Isaiah, we find the expression "God of truth" (literally "God of the Amen"), that is, the God who is faithful to his promises: "He who blesses himself in the land shall bless himself by the God of truth [amen]."
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.