The other thing I've often wondered about this is whether nobles being priests actually makes sense...Or rather nobles being something like Bishops and such would make more sense, having charge over NPC priests. Bishops and such often did fight and all...
I mean, "priest" is the general term. "Bishop" is specific to Christianity, so we're not going to use that.
There very much
are NPC priests of every religion—they're the ones running the temples, etc. PC priests are just much more effective, because when you've got a noble of the highest order preaching, it carries more weight than just any other random commoner.