So this joke was originally about two rabbis. With some tweaking, it can be a joke about Unitarian Universalists.
These two old Unitarian Universalists had been arguing for years about religion. They were both agnostics, and they would meet once a week, sit on a park bench, and go over the arguments for and against God. They had been meeting like this for fifty years, they had never made any progress in their arguments, but still they kept at it.
Finally, God got sick of hearing these two argue. So one week, God appears in front of their park bench and says, “I can tell you the one logically valid argument for God’s existence, because I’m God.”
“Look, pal,” says one of the old Unitarian Universalists, “I don’t care who you are, go away and don’t spoil our fun.”