The Collaborative International Dictionary of English v.0.48:

Unbelief \Un`be*lief"\, n. [Pref. un- not + belief: cf. AS.
   ungele['a]fa.]
   1. The withholding of belief; doubt; incredulity; skepticism.
      [1913 Webster]

   2. Disbelief; especially, disbelief of divine revelation, or
      in a divine providence or scheme of redemption.
      [1913 Webster]

            Blind unbelief is sure to err,
            And scan his work in vain.            --Cowper.
      [1913 Webster]

   Syn: See Disbelief.
        [1913 Webster]

WordNet (r) 3.0 (2006):

unbelief
    n 1: a rejection of belief [syn: unbelief, disbelief] [ant:
         belief]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "unbelief":
   apprehension, atheism, disbelief, distrust, doubt, dubiety,
   faithlessness, gentilism, incredulity, infidelity, minimifidianism,
   misgiving, mistrust, nullifidianism, qualm, secularism, skepticism,
   suspicion, unbelievingness, uncertainty, unfaith