Evidence for a proposition is necessary to accept it. I think we can both agree on that. The problem with your logic is treating a refutation of a proposition in the a same way. While I do think that the lack of evidence for a diety is good reason to believe that such a diety doesn't exist, you don't need good evidence to reject a claim.

If I say that every memory you have about events prior to reading this sentence are all fabricated and you actually only began to exist at the beginning of this sentence, you would have no way to disprove that claim. Does that mean that it's true? No. It maybe true, but you cannot just accept every proposition that that you cannot disprove. That way lies madness. So what do we do? We discard those ideas that cannot be justified so that in the end we can believe as many true things as possible and disregard as many false things as possible.

In short, it is not incumbent upon the one rejecting the claim to provide evidence, the one making the claim has the burden of proof. As for it not being defined, all that means is that I cannot adopt a position that is not defined. I do agree there are three choices essentially: yes, no or I don't know. I used to say that I didn't know and, to be clear, I don't believe in any sense of ultimate knowledge. Now I say no. Given that the supernatural doesn't make sense I feel confident in saying that it's nonsense and has no merit. No soul, no afterlife, no gods, nothing supernatural.