Why do we profess the belief that there is only one God?

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We profess the belief that there is only one God because he has revealed himself to the people of Israel as the only One when he said, “Hear, O Israel: the Lord your God is one Lord” [Deuteronomy 6:4], and “there is no other” [Isaiah 45:22]. Jesus himself confirmed that God is “the one Lord” [Mark 12:29]. To profess that Jesus and the Holy Spirit are also God and Lord does not introduce any division in to the one God.

Ref. CCC 200-202, 228

Taken From The Compendium – Catechism Of The Catholic Church