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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "demonism":
   Black Mass, Druidism, Parsiism, Sabaism, Satanism, Zoroastrianism,
   animal worship, black magic, chthonian worship, demonography,
   demonolatry, demonology, demonomancy, demonomy, demonry,
   devil lore, devil worship, devilry, diablerie, diabolism,
   diabology, diabolology, fetishism, fire worship, heathenism,
   hero worship, iconoduly, iconolatry, idol worship, idolatrousness,
   idolatry, idolism, idolodulia, idolomancy, image worship, paganism,
   phallic worship, phallicism, pyrolatry, snake worship, sorcery,
   star worship, sun worship, the black art, tree worship