Catechism of the Catholic Church §1064 — excerpt
Thus the Creed's final "Amen" repeats and confirms its first words: "I believe." To believe is to say "Amen" to God's words, promises and commandments; to entrust oneself completely to him who is the "Amen" of infinite love and perfect faithfulness.
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.
Also cites: St. Augustine, Sermo 58, 11, 13: PL 38, 399