Those are good questions to ask, but I wonder if you think they only need to be answered by religious people. If someone doesn't believe in some particular god, it is still possible to ask them "Is this other person who believes in that god wrong?" and "How do you know?".
Perhaps a helpful example for showing this is the conspiracy theory concerning Bielefeld in Germany[0]. Is non-belief in Bielefeld any less subject to questioning than belief in it?
Anyway, keep looking for answers, and I wish you a good day too.
If one has read the Bible it becomes pretty apparent that YHWH, the God of the Bible, claims (and I believe is) the only God and that all other gods essentially aren’t real.
A god is usually thought of a some deity but the meaning is much deeper than that: a god is something one worships. Many people worship money as god, their job or company as god, drugs or sex as god.
But like I said, the Bible makes it very clear that there is only one God (according to the Bible).
