The Collaborative International Dictionary of English v.0.48:

Unbeliever \Un`be*liev"er\, n.
   1. One who does not believe; an incredulous person; a
      doubter; a skeptic.
      [1913 Webster]

   2. A disbeliever; especially, one who does not believe that
      the Bible is a divine revelation, and holds that Christ
      was neither a divine nor a supernatural person; an
      infidel; a freethinker.
      [1913 Webster]

   Syn: See Infidel.
        [1913 Webster]

WordNet (r) 3.0 (2006):

unbeliever
    n 1: someone who refuses to believe (as in a divinity) [syn:
         disbeliever, nonbeliever, unbeliever]