The Collaborative International Dictionary of English v.0.48:

Atheistic \A`the*is"tic\, Atheistical \A`the*is"tic*al\, a.
   1. Pertaining to, implying, or containing, atheism; --
      applied to things; as, atheistic doctrines, opinions, or
      books.
      [1913 Webster]

            Atheistical explications of natural effects.
                                                  --Barrow.
      [1913 Webster]

   2. Disbelieving the existence of a God; impious; godless; --
      applied to persons; as, an atheistic writer. --
      A`the*is"tic*al*ly, adv. -- A`the*is"tic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

atheistic
    adj 1: rejecting any belief in gods [syn: atheistic,
           atheistical, unbelieving]
    2: related to or characterized by or given to atheism; "atheist
       leanings" [syn: atheist, atheistic, atheistical]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "atheistic":
   apostate, atheist, backsliding, blasphemous, disbelieving,
   faithless, fallen, fallen from grace, gentile, goyish, heathen,
   impious, infidel, infidelic, irreligious, irreverent, lapsed,
   minimifidian, misbelieving, non-Christian, non-Jewish,
   non-Mohammedan, non-Mormon, non-Moslem, non-Muhammadan, non-Muslim,
   nullifidian, pagan, profanatory, profane, recidivist, recidivistic,
   recreant, renegade, sacrilegious, unbelieving, unchristian,
   uncircumcised, undutiful