OK, I think this is an old one, but ...
Four rabbis meet up every weekend to sit and chat, discussing theology. Every time they talk, it ends up with three of them agreeing against the other, whatever the topic of the day, and regardless of who's really right. They've just got it in for this guy, apparently, and he always ends up on his own against the three of them.
One day, he finally gets fed up with it. He knows he's right about that day's debate, but the other three stick to their guns and stick together against him. Exasperated, the lone rabbi resorts to prayer:
"Oh God," he prays, "I know I'm right here, and you, as the omniscient Supreme Being, must know I'm right, too. Please send a sign to show these three idiots that not only am I right, but that they're going up against the Almighty as well. Just a little sign, Lord, that's all I ask. Amen"
Suddenly, out of a clear blue sky, a cloud appears above the heads of the three, and starts raining on them for a few seconds, then disappears.
"Ha! You see?" cries the lone rabbi. "A sign from the Almighty to show you you're wrong and I'm right! What do you say to that?"
"Meh," comes the reply. "An unusual weather phenomenon, nothing more. Easily explained by natural causes."
The lone rabbi decides to try again. "Oh Lord," he prays, "apparently that sign wasn't enough to convince them. They still say I'm wrong, three to one. Can you send a clearer sign so they'll understand? Amen."
Again, a cloud appears, darker than the first. It hovers over the three rabbis and, one by one, hits each of them wth a small jolt of lightning, just enough to curl their hair without doing any lastng damage. The cloud then changes shape to spell the words "he's right", before disappearing in a "poof!", leaving the clear sky once more.
"Now will you admit I'm right?" asks the lone rabbi. "Was that sign clear enough for you?"
"Well," they reply, "we have to admit that was very, very unusual, but still, it's probably explainable by natural causes despite its curious nature. We still think we're right, and it's still three against one."
"Oh for fuck's sake!" thinks the rabbi, not a little peeved at their obstinacy. He offers up one last prayer: "Dear God, it's really getting annoying. Two signs you've sent them, and twice they've ignored you. Could you send a really big sign, something that can't be mistaken for a natural phenomenon, and show them just how wrong they are? Amen."
In an instant, a rip appears in the sky, replacing part of the blue with a curtain of the deepest black that covers half the sky, out of which rolls a magnificent white cloud in the shape of an old, bearded man. A deep, booming, but exasperated voice rolls from the cloud, saying, "He's right and you three are wrong. Do not try my patience further. Accept this sign or I willl send a greater, less friendly sign to ensure you doubt me no further. He's right. Now give me some peace and quiet."
"Aha!" says the lone rabbi. "Now what do you have to say?"
The three rabbis look at each other long and hard, shaken by the experience and not sure how to react. Finally, they nod in silent agreement, as if they had telepathically consulted on what their answer should be. One of them looks the lone rabbi square in the eye and says,
"Fair enough. Now it's three against two ..."