I like the idea of H/P bumps tied to conversions, and instead of friendly troops not stopping a claim region for religion attempt, see below "Assert Divine Right".
More offensive options...
"Exhort the faithful against their lord"
When used in a region that has a lord not of your faith, it encourages the followers of your religion to resist the control of the lord (and by extension, realm). Control is lowered proportionally by the percentage of followers in the region, and once beyond a certain minimal percentage, there is an increasing chance of a rogue militia unit forming to actively fight.
This would be an option that doesn't require the absolutely dominating percentage of followers that some of the other options do, could be very useful in holy wars, and fits with what Priests in BM are supposed to be able to do and what priests in history could do.
"Assert divine right: Purge the heretics/Protect the faithful"
Priest options to assist in takeovers. Both fear and sympathy based options, and I would suggest having their impact on followers tied to the success of the TO. If the TO succeeds, then the percentage of the succeess due to religious options equates to percentage increase in followers (clearly, you were right), and similarly, if the TO fails it causes losses (clearly, you were wrong). H/P gains/losses should similarly happen. Priests are very publicly putting their reputation and influence on the line for this.
"Repurpose shrines/temples"
You should be able to convert shrines (percentage chance the shrine is just destroyed) and temples (percentage chance temple loses one or more levels) of other faiths. Likely to cause a religious riot, friendly troops can help suppress. Serious historical accuracy here, the number of shrines/gods/temples that Christianity in particular assimilated is vast, and the Hagia Sophia is the most prominent example I can think of.
"Call for martyrs"
Fill RC's faster. Very straightforward, also accurate (allowing for how BM recruitment works).
