Nero's fire said:The idea of "Defenders of the Faith" could reduce war
I am not familiar with any other mod that moves holy cities and I see this being a considerable challenge for a small thing. Like a great scientist with writing the holy city is the reward you get for getting to the way tech first.
This is designed as a discouragement from hording religions. If you found a religion and adopt you will not lose the holy city, if you start to get greedy and found more than one, expect to lose the one you are not using.