The Collaborative International Dictionary of English v.0.48:

Theistic \The*is"tic\, Theistical \The*is"tic*al\, a.
   Of or pertaining to theism, or a theist; according to the
   doctrine of theists.
   [1913 Webster]

WordNet (r) 3.0 (2006):

theistical
    adj 1: of or relating to theism [syn: theist, theistical,
           theistic]