Terry Mattingly has it right. Those writing about religion would do best to follow the AP stylebook, which defines the term thus:
“fundamentalist: The word gained usage in an early 20th century fundamentalist-modernist controversy within Protestantism. In recent years, however, fundamentalist has to a large extent taken on pejorative connotations except when applied to groups that stress strict, literal interpretations of Scripture and separation from other Christians.
“In general, do not use fundamentalist unless a group applies the word to itself.”