Is there any way I can make it so that only certain civs can have a particular religion? I think it would be good to limit them to the civs where the religion was prominent. One possibility I can think of (though I don't know if it would work) is to prevent the civ from researching the prerequisite tech. I don't know if that would stop the spread of the religion into that civilization's cities. I've had a look through the tutorials, but couldn't see anything about this. Can anyone help?

This is clearly a tricky question then...has anyone tried modding religions in this way before?

How deeply do you want to go to make this happen? If you want to go into the SDK, you could add whatever checks you want in CvPlayer::canDoReligion(). If you are limiting yourself to Python... I really don't know if there's an easy way. I didn't see anything obvious in CvGameUtils unfortunately.

Note that if you have the "Choose Religions" game option set, they already are more likely to go after their defined Favorite Religion but I expect you want things to be much more limited.

I would like things to be much more limited. The Romans being a buddhist state? The Iceni discovering Confucianism? Definately not; I'd like to limit each religion to civilizations who traditionally embraced it.

Thanks for the suggestion though I don't think I have the skills (at the moment) to do it ;).