How about making H/P for priest depend on the power of their religion?
You could say a Priest with higher prestige has more followers. But that also implies that when the flock dwindles, a priest should lose prestige.
Also, in a religion, the institute has great value: priests higher in the chain (especially Elders) should always be more prestigious, even if they would do less for the faith then a regular member, for instance.
So how about giving any given religion a total amount of H/P (based on amount of followers), which is then distributed across the Priests (or even regular members) of the religion?
For example, SA has 200,000 followers (wild guess). This grants the religion a total of 1000 prestige. The cake is then divided across all layers of followers. The Founder of course gets the biggest chunk, closely followed by higher (and lower) Elders. Regular priests get less, and Aspirants get barely anything. Active promotoring nobles of the religion (non-priests) get a small bonus to their H/P, through the regular system (getting H/P for constructing a temple, for instance)
This would also allow for more internal strife, because your position directly influenced your H/P. Total H/P for any given continent would be constant, but the division between several religions would fluctuate.