The Collaborative International Dictionary of English v.0.48:

Disbeliever \Dis`be*liev"er\, n.
   One who disbelieves, or refuses belief; an unbeliever.
   Specifically, one who does not believe the Christian
   religion. --I. Watts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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