You ask the priest to ask his god if his god exists? The priests "cast a spell," asks the question, and then turns to you and says, "Yes, my god exists."
That's odd. I asked the hedge wizard down the road the same question, and he did the same thing and said, "No."
Bullgrit
For one thing, the rules don't tell us if the priest gets some sort of "out loud" answer or if its quietly whispered in his ear or divined from goat entrails, so how do we know? It could even be if it is an out-loud answer, the hedge wizard uses
magic mouth to produce the lie he wants a person to hear.
I can show you a video of the autopsy of an alien from Area 54. The majority of people believe that to be a hoax, but couldn't explain
how it's a hoax.
You can show me pictures of Africa, take me to the zoo and show me African elephants and I can turn to you and say, "I've never really been to Africa, how does this prove it exists?"
Poeple's acceptance of reality is often more often based on people's say so - and their documentation - than first-hand proof.
In the D&D game, the so-called proof is pictures, anecdotes, research or eye-witness accounts written down in holy texts. In one case, the holy text of a religion may be derived from the actual text a god wrote and disseminated to his followers - or via a prophet. Other texts may have "researched" via Augury or Contact Other Plane. An artist may have adventured to Hell and drawn images in his journal that's he brought back. Perhaps peasants have witnessed a portal being opened to hell and recount their experiences to others.
All of these would be factual information passed to others that could be collabrated by others. Many individuals would not have had their own such experiences, but just like we "know for a fact" that Africa exists and what it is like from the compiled, collaborated experience of others, those in the fantasy game would "know for a fact" about the afterworld from similar such collections and documentations.
In your example, it may be the word of one wizard's answer to the repeated answer of a hundred clerics. Does that make more sense?