The Collaborative International Dictionary of English v.0.48:

Unbelieving \Un`be*liev"ing\, a.
   1. Not believing; incredulous; doubting; distrusting;
      skeptical.
      [1913 Webster]
      [1913 Webster]

   2. Believing that the thing alleged is not true;
      disbelieving; especially, believing that the Bible is not
      a divine revelation, or that Christ was not a divine or a
      supernatural person. "Unbelieving Jews." --Acts xiv. 2.
      [1913 Webster] -- Un`be*liev"ing*ly, adv. --
      Un`be*liev"ing*ness, n.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unbelieving \Unbelieving\
   See believing.

WordNet (r) 3.0 (2006):

unbelieving
    adj 1: rejecting any belief in gods [syn: atheistic,
           atheistical, unbelieving]
    2: holding that only material phenomena can be known and
       knowledge of spiritual matters or ultimate causes is
       impossible [syn: nescient, unbelieving]
    3: 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:

42 Moby Thesaurus words for "unbelieving":
   atheist, atheistic, creedless, disbelieving, distrustful, doubting,
   faithless, gentile, goyish, heathen, heretical, incredulous,
   infidel, infidelic, irreligious, minimifidian, misbelieving,
   mistrustful, mistrusting, non-Christian, non-Jewish,
   non-Mohammedan, non-Mormon, non-Moslem, non-Muhammadan, non-Muslim,
   nonbelieving, nullifidian, pagan, questioning, quizzical,
   repudiative, sceptical, skeptical, suspicious, unchristian,
   uncircumcised, unconfident, unconverted, unconvinced, unpersuaded,
   without faith