The Jewish-Christian creed known as the Shema [Deu 6.4; Mar 12.29] contains within it the word “One”, a quantifier, simple numeral which helps the reader define how many God is.
The same is true when innumerable times in the Hebrew Bible One means a single Divine Lord, One Person, One divine Self.
Add to this the fact that this One God of Israel is defined in Scripture by thousands of singular personal pronouns, which as we all know, designate a single person.
Finally, the word “God” in the New Testament designates the Father of Jesus, whom he described as “the Only One who is true God” (John 17:3; cp. 5:44; Mark 10:18).