I just posted a message explaining the advantages of messing with religous values and other things. They are useful tools for us campaign writers but not always historically correct. IMO the best campaigns we write wont be historically correct because of this.

Heres the link. http://www.totalwar.org/ubb/Forum5/HTML/000779.html

Religious values are required for both a faction and a region. Cultural values are also required but I dont remember seeing a heretic culture.

The best you can do is possible use the pagan culture because i am not sure that a heretic culture even exists in the game.

Giskard