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]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "misbeliever":
   Albigensian, Arian, Cathar, Donatist, Ebionite, Erastian, Gnostic,
   Jansenist, Lollard, Manichaean, Monophysite, Montanist, Pelagian,
   Sabellian, Waldensian, Wyclifite, antinomian, dissenter, dissident,
   emanationist, heretic, hylotheist, nonconformist, pantheist,
   schismatic, sectary, separatist