25h, savegame attached. I just completed Purge the Unfaithful the turn before I saved, when I completed it, almost all of my cities went into revolt. To my knowledge I had very little outside religion, and I know that any I had wasn't there nearly as long as my own religion. So seems like a bug to me. I am pretty sure I attached the quicksave as well. It should be a fair ways earlier in the game, but with the same basic city layout, so can give an idea of what religions were already in place.