Chapter 1 — Preface to the Roman Catechism
Section 23. Division of this Catechism — The truths revealed by Almighty God are so many and so various that it is no easy task to acquire a knowledge of them, or, having done so, to remember them so well as to be able to explain them with ease and readiness when occasion requires. Hence our predecessors in the faith have very wisely reduced all the doctrines of salvation to these four heads: The Apostles' Creed, the Sacraments, the Ten Commandments, and the Lord's Prayer.