I was thinking clerics should have some sort of incentive to convert others to her faith. Specifically, I was thinking that a cleric who begins the game as the founder and prophet of her faith should get XP for converts and for building and sanctifying shrines.
Convert Person and Mass Convert
Perhaps she gets XP equivalent to slaying a monster of equal level to a convert. Would that be enough? Instead of a difficult mechanic, look up your edition’s version of charm person and mass charm, scratch out charm and write in convert. Instead of adding it to the cleric’s spell list, make it a once-per-month ability and make it non-magical. Convert is possible at level 1, Convert Monster at level 4, Mass Convert Person at level 7 and Mass Convert Monster at level 9. Converts’ spiritual allegiance is to the god or philosophy or whatever they have been converted to and the cleric is seen at the focus of this new faith. Have the convert make a periodic saving throw as if for a charm spell. If they make it, then they are no longer ‘charmed’ by the cleric, but still regard her highly. Their charmedness has converted to their new god or faith. Faith can certainly be lost in stressful or calamitous situations, but there’s no need for a mechanic for that unless the GM wants to allow a saving throw vs the conversion when some tragedy strikes.
Converting Stannis’ wife was a start. Converting Stannis was a coup.
Shrines and Holy Places/Fixtures
Building a shrine or holy fixture like a stupa is a major undertaking. In the interests of not coming up with yet another system, use the magical enchanting or research rules of your favorite edition. The cleric declares the magical item equivalent or spell research equivalent of this shrine. When completed, she gets the value of that magical item in XP or the monetary cost of the research aspects of shrine and all its components as XP, provided she hires guards and provides for its upkeep in advance for at least a year. The upkeep costs do not convert to XP. If the shrine is lost, the XP value is immediately taken and could lead to a loss of level similar in effect to a level drain.