Maybe you just didn't take it FAR enough.

Forget the Immortals having their power raised or lowered based on the number of mortals they can "Pray for me, and you'll get a 10% kickback... act now!" into their religion.

Set up the faith points, but rather than have it affect the Immortal have it affect a mortal "avatar", or champion, of sorts. Don't make it a method of delivering ultra-powerful abilities, but rather ultra-cool. Very roleplay oriented stuff, but with a small little tweak that's small enough that you wouldn't miss it, yet enough to say, "neat!" over.

The faith points work like mana, but they also work as a booster. The avatar gets a SMALL bonus to his supplications while the faith level is high (I'm not giving any numbers here, since I don't know how the scale would work), or he could use up the faith points to execute those fancy abilities (revolving around the particular faith). So, the more (s)he uses the abilities, the less of a boost (s)he gets. At it's highest point, you wouldn't be able to use the special trick more than, oh... 3 times.

Now, occassionally the Immortal would be allowed to give a FP boost (up to the "normal" FP limit) to their Avatar, but it would do an echo to all the currently active Immortals about what had happened, and another Immortal would be able to respond similarly with their own champion/avatar. The drawback to this being the mortal's life. When that boost wears off, he dies. Not for good, but a death. Consider it the power being channeled just burning him/her up from the inside. The mortal in question gets a flag on his character that doesn't allow "boosting" for a LOOOOOOOOOOONNNNNNNNNGGGGGGGGG time. As long as that flag is there, nobody else can be named avatar/champion, and they can't be boosted again. And have the process of naming a champion/avatar exhaust all FPs.

I just see this all adding an incredible cool "(un)holy war" type atmosphere to what currently just kinda feels like belonging to, well... if cabals are like clubs, then religions are kinda like unions. And let's be honest... it's pretty much a truism of mythology that the gods rise and fall on the worship of their followers, whether it be directly (harvesting the energy of their belief) or indirectly (hard to be a god when your followers are made extinct).