Definition of proof courtesy Merriam-webster...
"the cogency of evidence that compels acceptance by the mind of a truth or a fact"
The joke here is that no evidence will pursuade someone that God exists, because someone who doesn't believe in the truth doesn't have any standard for evidence other than personal whim.
The problem is not that the unbeliever lacks evidence. They are surrounded by evidence. The problem is an internal one. It is a heart issue.
When the God denier demands proof, they are simply reveling in their arbitrariness. There is no proof they will accept as valid, because they will always have doubts.
Yet it is their doubt itself that even proves God, because the one who claims to know nothing at least knows that they have doubts. It is true that they have doubts. So even those who claim to know nothing believe that there is truth.
And what is God? The Most Real. The Ultimate Reality. If you believe in truth at all, you believe God exists whether or not you acknowledge God as being God.