The Catechism in Questions & Answers

53. What does it mean when we say: “God is the One God”?

There is only one God. Belief in the One God is a fundamental profession of the Old and New Testaments, and is thus also fundamental to the Christian faith.

‘Monotheism’ is the designation for the doctrine that there is only a single God. Monotheistic religions include, for example, Christianity, Judaism, and Islam.

“Thus says the Lord, the King of I srael, and his Redeemer, the Lord of hosts: ‘I am the First and I am the Last; besides Me there is no God.’” Isaiah 44: 6
“Hear, O Israel, the Lord our God, the Lord is one...” Mark 12: 29