Catechism of the Catholic Church §875 — excerpt
"How are they to believe in him of whom they have never heard? and how are they to hear without a preacher? and how can men preach unless they are sent?"
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: Romans 10:14:15