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]