The Collaborative International Dictionary of English v.0.48:

Disbelieve \Dis`be*lieve"\, v. t. [imp. & p. p. Disbelieved;
   p. pr. & vb. n. Disbelieving.]
   Not to believe; to refuse belief or credence to; to hold not
   to be true or actual.
   [1913 Webster]

         Assertions for which there is abundant positive
         evidence are often disbelieved, on account of what is
         called their improbability or impossibility. --J. S.
                                                  Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disbelieving
    adj 1: denying or questioning the tenets of especially a
           religion; "a skeptical approach to the nature of
           miracles" [syn: disbelieving, skeptical, sceptical,
           unbelieving]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "disbelieving":
   atheist, atheistic, creedless, faithless, heathen, heretical,
   incredulous, infidel, infidelic, irreligious, minimifidian,
   nonbelieving, nullifidian, pagan, questioning, quizzical,
   repudiative, skeptical, unbelieving, unchristian, unconfident,
   unconverted, unconvinced, without faith