The Collaborative International Dictionary of English v.0.48:

Diabolism \Di*ab"o*lism\, n.
   1. Character, action, or principles appropriate to the devil.
      [1913 Webster]

   2. Possession by the devil. --Bp. Warburton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "diabolism":
   Black Mass, Satanism, black magic, chthonian worship, demonism,
   demonography, demonolatry, demonology, demonomancy, demonomy,
   demonry, devil lore, devil worship, devilry, diablerie, diabology,
   diabolology, sorcery, the black art