Catechism of the Catholic Church §2558 — excerpt
"Great is the mystery of the faith!"
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. Therese of Lisieux, Manuscrits autobiographiques, C 25r