The story
From heathen + -ism, coined in the 17th century in place of the then-obsolete heathendom. First attested in Francis Bacon's Of the advancement of learning (1605), II. xiv. §9: "The heresy of the Anthropomorphites … and the opinion of Epicurus, answerable to the same in heathenism, who supposed the gods to be in human shape" (OED).
Etymology from Wiktionary, CC BY-SA 4.0.