http://www.twcenter.net/forums/showt...63#post8905163
Use "heretic" as the religion name. I just tried it out with England. It works, but I'm not sure how it will go in the long term. Heresy also seems to spread very fast. Without a religious building in London, heresy went from 2% to 21% in the first turn.
As far as getting crusades called on you, you'd have to make sure that your settlements are potential crusade targets in data/world/maps/base/descr_regions.txt
And I'm not sure if it would be possible to recruit Heretics. I'll look into that.