Restrictions on a certain number of elders being required to be of the priest class can sometimes make it difficult, though. But yes, obviously it would not work to have the priest class as it exists now be able to lead troops. I'm just saying that realistically, priests sometimes did. And I doubt it affected their ability to give sermons.
And if we were going by realism, and not game balance, you wouldn't need a certain number of elder priests to keep a religion going, and any character that's trained up their infiltration skill could use it any time, and you could actually kill other characters, and...
You begin to see my point?
Also, if you're worried about changing class because of the elder priests restriction, your religion is probably already in deep trouble.
Ideally, I think, there would be some separate class that counted as a priest for elder rank purposes, could lead troops (maybe less than normal, for balance), traveled as a normal warrior, and had limited preaching options. But I don't really expect such a thing to be added any time soon.
Nope, ain't gonna happen.