Whenever I take over an enemy capital that was a Holy City for another religion, I always convert it to my religion and remove the other religion from it completely.
Sometimes I've noticed that after a few turns, inexplicably, the original religion will spontaneously gain 1 follower, which is enough to give it 10 pressure even though it's not a majority religion.
What causes this? I always make sure to also clear all surrounding cities of the original religion (at least I make sure that my religion is the majority in all cities, meaning that the other religion shouldn't spread). Is it caused by having even one religious follower of the old religion somewhere?
Best Answer
A holy city can only be prevented from regaining a religion if the religion is destroyed. If it isn't, the city will be affected by internal pressure, usually 30. For the religion to be destroyed, there can not be a single city with that faith as its dominant religion. The holy city will continue to become holy again unless you do one of three things.
Option one: Station an inquisitor in the city. Problem solved.
Option two: Find every city with that faith as the dominant religion and change it.
Option three: Surround the holy city with enough cities of your faith so that the internal pressure is not enough to convert it back. This is probably 5, since internal pressure is usually 30.
PS: Don't ask me why internal pressure isn't always 30, I am not sure. It may regenerate over time after an inquisitor or missionary? I have seen it below 30 but never above.