The term "religious fanatic" can be applied to anyone whose strength of belief is distasteful to others. One person's saint is another's pariah.
So far as I'm aware, the term is used exclusively as an accusation, a derisive means of saying that the believer has gone too far. As such, they are thought to be at the least liable to twist your arm to win you to their convictions or to be dangerous. Some labeled as such have, in fact, done such things.
But in a society like America, where a person's religion isn't supposed to stick out too much lest it upsets the egalitarian ideal, someone can be called a fanatic simply for taking religion seriously. Those who take St. Francis or Gandhi or Mohammed as role models stand a good chance of being shoved to the margins of society because they don't know when to stop being religious, unlike most citizens who know when to quit in a pragmatic sort of way.