There are first hand accounts such as ‘Proof of Heaven’, written by a formally atheist neurosurgeon, but of course I doubt you’d accept that as proof. We’ll know when we die, but that’s no use to the living. My point is it is impossible to provide any other kind of evidence.

Interesting definition — the word itself, ‘a’ (non) theist (believer in a divine being or beings) sounds pretty concrete to me but of course it is up to the individual how they define their beliefs.