What if a human being simply rejects the theory that God exists?
Then they'd be rejecting their own existence... given that God created them and the universe they exist in
Circular reasoning. As your evidence and your conclusion is the same thing.
Worse, your own logic doesn't support such an idea. As you've forwarded a 'first mover' argument. A first mover argument doesn't require a God. It requires a force that moved first. That's its sole and defining characteristic
All the other attributes of this first mover, from sentience, to talking to us, to being all powerful, to designer, to intentionality, to commandments, to personality, to being worthy of worship....
none of that is supported by your first mover argument.
Thus, you have no logical or rational basis for your own claims, by your own standards.
What if that same human being via his own ability to reason decides upon a perfectly acceptable set of moral values, based on reason
Then they'd be using the reason endowed to them by God.
And who says that 'god' endowed anyone with anything? Your 'first mover argument doesn't say a thing about 'reason' or 'endowment'. Only that something moved first.
The rest, you made up.
But only because such he's presenting an idiosyncratic belief or impression that is firmly maintained despite being contradicted by what is generally accepted as reality or rational argument, typically a symptom of mental disorder, OKA: Delusion.
The obvious problem being.....you can't demonstrate any characteristic of your 'god', nor factually verify anything you've said.
You assume it all.
And refusing to accept a series of nested assumptions that you can't back factually isn't a 'mental disorder'. Its the application of fact-based empirical reasoning.
Even logically, your claims are ridiculously unlikely. As there are many, many different religious beliefs. And almost all religious beliefs are mutually exclusive. It can't be Jesus AND the Greek Pantheon of Gods. Nor does it have to be either. Which leads us to one of three probable conclusions:
1) There is no god
2) There is a god, but none of us know his will.
3) There is a god, and one of the thousands of mutually exclusive religions knows his will...
which means that all others got it wrong.
The 1st and 2nd option render your entire perspective moot. So your best case scenario is option 3. And its still not a good one for you. If one religion got it right, that means that all the other religions are wrong and self deluded. And there are thousands upon thousands of different religions.
Thus
its ridiculously unlikely that in all the world, of all religions, in the entire history of our planet.....
that YOU happened to be the one to get it right.
Meaning...
.its orders and orders of magnitude MORE likely that you're among the incorrect and self deluded masses. Where almost all of the religious that ever lived would have to fall using your own standards.
Logically, your claims are crap.