Chapter 11 — Part I: The Creed — Article IX: The Holy Catholic Church; the Communion of Saints
Section 28. "One' — The first mark of the true Church is described in the Nicene Creed, and consists in unity: My dove is one, my beautiful one is one. So vast a multitude, scattered far and wide, is called one for the reasons mentioned by St. Paul in his Epistle to the Ephesians: One Lord, one faith, one baptism.