Sisiutil
All Leader Challenger
I've suggested something like this in the past, as an option to get rid of the "religious spies" that you have in your cities. There should be a price, of course: loss of population seems to be the obvious one, and without any production benefit like you get from slavery. All the buildings for that religion are destroyed as well. (This is on top of the other costs--you can't build additional monasteries, and if you run FR later, you don't have that religion around for its happy benefits.)Actually, rather than atheism (which has only really been at all significant in the last century), what I'd be interested in seeing is some sort of inquisitor-type unit that can purge a religion from a city.