But there are so many messengers, and no test for which ones got it right and which didn't. This is underlined by the way they disagree with each other.
With respect, no, we don't.
Put it this way: what test should I apply to:
this keyboard I'm typing on
that woman there waiting for a bus
the sun
to tell whether any of them is God / a god or not?
If God is real, what is godness, the real quality that makes God God / a god, even when [he] might take the form of a keyboard, that woman, the sun? What quality that exists independently of anyone's imagination is it that we're testing for?
Start with the quality 'unknowable' ─ that compels the conclusion that all claims about God's nature are imaginary, doesn't it?
And again correct me if I'm wrong, but 'eternal', 'omniscient', 'omnipresent', 'almighty', and 'transcendent' aren't properties of anything or anyone demonstrably real that we know of, are they? They're absolutes, aspirations, and (in this context) above all abstractions, no?
We won't know that till we see it, though.
If [he] has no physical form then he's not real. And the only alternatives to being real are being imaginary (in which I include concepts with no real counterpart) and not existing at all, no?
I dislike the idea of blasphemy, ideas you're not allowed to think. In my view, all things are open to reasoned enquiry.