Update: I think the direct answer to your stated question would be yes, a priest will change his belief assuming (a) the man is rational and not delusional, and (b) the proof is truly beyond doubt. I'd be surprised if that were the case. (Of course, it's a hypothetical question). Priests are not stupid - they know that the minute God is "proven", the game changes for them, and they can no longer play it "deuces wild". Proof of God would likely wreck religion.