I know how to change a factions religion (with still a few really minor snags) from the start of the game.
I think that I've also figured out how to add new religions, although this still needs testing.

What I am looking for now, is for a way to script the change of a factions religion in game if it's possible at all.

Reformation, a certain English King, the schism of the catholic church (into "three" groups, Byzantine Orthodox, Russian Orthodox and Catholics)

Anyways, it would be lovely if someone could help me find that ?

Dingo