Do citizens change religion once they've converted to one or another?
For instance, if I have a border city, with 5 followers of Christianity, and 4 followers of Islam, and 9 population, is there any way this city will flip to Islam if there are no inquisitors or great prophets used?
I was playing a Duel map against Gandhi, and he only had one city. I sent in a Great Prophet and completely flipped all 12 citizens to my religion, and I thought this would kill his religion, since the Holy City converted to my religion, but a few turns later he not only had flipped all the way back, but had sent a missionary my way. How did he do that?