Source : Webster's Revised Unabridged Dictionary (1913)

Misbeliever \Mis`be*liev"er\, n.
   One who believes wrongly; one who holds a false religion.
   --Shak.

Source : WordNet®

misbeliever
     n : a person who holds religious beliefs in conflict with the
         dogma of the Roman Catholic Church [syn: {heretic}, {religious
         outcast}]