The Collaborative International Dictionary of English v.0.48:

Demonism \De"mon*ism\, n. [Cf. F. d['e]monisme.]
   The belief in demons or false gods.
   [1913 Webster]

         The established theology of the heathen world . . .
         rested upon the basis of demonism.       --Farmer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

demonism
    n 1: a belief in and reverence for devils (especially Satan)
         [syn: diabolism, demonism, Satanism]