We Christians believe there is only one God. It must be the same only-one-God as that of the Jews, because Christianity came from Judaism, and Jesus was a Jew. Their bible is our Old Testament.
The Muslims also believe in only-one-God. They acknowledge that Abraham and Jesus were prophets of this God and only insist that Muhammad was later; the last and most recent prophet of this same God. They do not, however, acknowledge the triune nature of the God; Father Son, and Holy Spirit. In this, they are like Unitarians and Jehovah’s Witnesses, whom no one questions as to their being Christian.
It’s hard for me to see that Jews, Christians and Muslims are not worshipping the same God, despite doctrinal differences. Pope Benedict thought it was the same God. So did George Bush II, the previous president of the USA. I agree.
It’s an interesting question. As I recall, in the Book of Job, there’s a discussion between Yahweh and Satan, with the latter egging the former on to check out Job’s faith. But as I recall this took place at the assembly of gods, so while Yahweh was greatest, he wasn’t unitary. And there are some other places where the word “gods” turns up including the first commandment. In Exodus 18:11, for example, “now I know the Lord is greater than all the gods…” numbers 14:9 “the Lord is with us and has defeated the gods who protected them…” Scan a Bible app for “Gods”, and you get many many examples.
So I suppose one could argue there is potentially more than one God, or that people thought so, while insisting there was only one. I think the Gnostics believed our God was a bit player in the greater cosmos, run by a bigger God somewhere. So the idea of multiple gods is not uncommon and I suppose each one would want its supporters to see him/her as the One and Only.