There is a line between the devout and the fanatic and the greatest tragedy is the fanatic thinks he hasn't crossed it.
Read "Fanatic" by Eric Flint. It can be found in the anthology "The Service of the Sword" by David Weber ISBN: 0-7434-3599-0
I figure once you become devout, it's only a matter of degree.
After all, it's the same mentality, just amped up a bit.