The Collaborative International Dictionary of English v.0.48:

Misbeliever \Mis`be*liev"er\, n.
   One who believes wrongly; one who holds a false religion.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

misbeliever
    n 1: a person who holds religious beliefs in conflict with the
         dogma of the Roman Catholic Church [syn: heretic,
         misbeliever, religious outcast]