Nowadays, when you ask an AI to adopt your state religion, he/she might accept or decline, but will almost always switch back to it's own religion after a couple of turns. Instead I think that adopting a religion as a term of peace would work as a kind of capitulation in Warlords/BtS, and make it impossible to change religion after that in the game. But there should be some kind of exception, like when a third part attacks your "vassal" and force it to change religion once again.
It would definitely make religion wars more interesting!
It would definitely make religion wars more interesting!
