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]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "unbeliever":
   Pyrrhonist, Sabbath-breaker, apostate, atheist, backslider,
   blasphemer, deserter, disbeliever, doubter, doubting Thomas,
   gentile, giaour, goy, heathen, infidel, kaffir, minimifidian,
   non-Christian, non-Jew, non-Mohammedan, non-Mormon, non-Moslem,
   non-Muhammadan, non-Muslim, nonbeliever, nullifidian, pagan,
   recidivist, recreant, renegade, sacrilegist, secularist, shegets,
   shiksa, zendician, zendik, zendikite, zetetic