If you accept the Merriam-Webster definition, the term "fanatic" is in fact a specific reference to religion.
So perhaps the term "religious fanatic" is straight from the "Department of Redundancy Department"
I wonder if reasonable Muslims make the same distinction between regular Christians and "fanatical" ones. Might be kind of tough considering the long-standing Christian support of Israel and the current action in Iraq. They might be subject the same lack of objectivity we had towards Muslims following 9/11.
Main Entry: fa�·nat�·ic
Pronunciation: f&-'na-tik
Variant(s): or fa�·nat�·i�·cal /-ti-k&l/
Function: adjective
Etymology: Latin fanaticus inspired by a deity, frenzied, from fanum temple -- more at FEAST
: marked by excessive enthusiasm and often intense uncritical devotion <they're fanatic about politics
