Catechism of the Catholic Church §228 — excerpt
"Hear, O Israel, the LORD our God is one LORD..." (Dt 6:4; Mk 12:29). "The supreme being must be unique, without equal. . If God is not one, he is not God" (Tertullian, Adv.
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.