Chapter 11 — Part I: The Creed — Article IX: The Holy Catholic Church; the Communion of Saints
Section 38. The Church is called holy because she is consecrated and dedicated to God; for so other things when set apart and dedicated to the worship of God were wont to be called holy, even though they were material. Examples of this in the Old Law were vessels, vestments and altars. In the same sense the first-born who were dedicated to the Most High God were also called holy.