If you toggle off FOW and look around the map, you'll probably see few (or no) priests and lots of heretics. At least that's what I've found in my games.
There are a couple of ways to approach this in descr_campaign_db.xml.
First thing that sticks out is these three entriesin the religion section:
Code:
<max_heretics_per_region uint="1"/>
<max_heretics uint="80"/>
<convert_to_heretic_base_modifier float="3.0"/>
<convert_to_heretic_unorthodox_modifier float="0.7"/>
The max_heretics is total amount of heretics allowed on the strat map. Lowering it will help.
The two convert_to_heretic control chances of priests to convert to heretics. Lowering them should also help.
In the Americas campaign, for example, all the above values are set to 0 and eliminates heretics.