Meditative techniques. Not foolproof, but “public” and widespread enough for us To know that there is something beyond what we call “normal” experience which most of us call “Divinity.” “Deities” are metaphors. I’m not talking about a narrow, arbitrary definition of “God.”
On the contrary, meditative techniques are good for showing what the brain can do. And it can certainly make up things under some conditions. They hardly show anything outside of the mind, though.
Normal experience is just that: what we commonly experience. is there something beyond normal experience? Of course there is. Drugs can give such experiences, as can meditation, or extreme physical exertion.
The question is whether these experiences are simply the brain deluding itself or whether they actually say something about reality (other than that the brain can delude itself).
What do you mean, 'Deities are Metaphors'? That, to me, justifies saying they are fictional: that they are a literary device. But I suspect that isn't your intent.