I found this info in another thread:
Quote Originally Posted by SilentResident
The following codes must be added ONLY in the Religion Buildings (Christian/Muslim).
{RELIGION_LEVEL}Bonus of the population's religion conversion
{AMPLIFY_RELIGION_LEVEL}Doublicates by % the Bonus of the population's religion conversion
{AGENT_LIMIT PRIEST}Increases the limitation in recruiting of Priests
Judging from this, it appears you'll need to up the conversion rates of the religious buildings, as nothing else apparently works. The only real difference is you'll need to make sure at least a basic small church is in each of your settlements. Otherwise, the idea should be exactly the same.