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):

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