The Collaborative International Dictionary of English v.0.48:

Heathenism \Hea"then*ism\ (-[i^]z'm), n.
   1. The religious system or rites of a heathen nation;
      idolatry; paganism.
      [1913 Webster]

   2. The manners or morals usually prevalent in a heathen
      country; ignorance; rudeness; barbarism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heathenism
    n 1: any of various religions other than Christianity or Judaism
         or Islamism [syn: paganism, pagan religion,
         heathenism]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "heathenism":
   Druidism, Gothicism, Parsiism, Sabaism, Zoroastrianism,
   age of ignorance, allotheism, animal worship, animatism, animism,
   barbarism, benightedness, benightment, dark, dark age, darkness,
   demonism, demonolatry, devil worship, fetishism, fire worship,
   heathendom, heathenry, hero worship, iconoduly, iconolatry,
   idol worship, idolatrousness, idolatry, idolism, idolodulia,
   idolomancy, image worship, pagandom, paganism, paganry,
   phallic worship, phallicism, pyrolatry, savagery, snake worship,
   star worship, sun worship, tree worship, unenlightenment