If it really is undecidable, God must be able to prove that.
However, I think an easier way to establish whether something is just your hallucination or a real (divine) being is asking them about something you couldn’t possibly know about and then check if it’s true.
Or: it says “This is undecidable in Zermelo-Fraenkel set theory plus the axiom of choice”. In the case of P=NP, I might believe it
I would not believe a purported god if it said all 9 remaining Clay math prize problems are undecidable.
If it really is undecidable, God must be able to prove that.
However, I think an easier way to establish whether something is just your hallucination or a real (divine) being is asking them about something you couldn’t possibly know about and then check if it’s true.