Catechism of the Catholic Church §2365 — excerpt
Fidelity expresses constancy in keeping one's given word. God is faithful. the Sacrament of Matrimony enables man and woman to enter into Christ's fidelity for his Church.
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. John Chrysostom, Hom. in Eph. 20, 8: PG 62, 146-147