Etymologie (noun.) before 12th century. From heathen plus -ism, coined in the 17th century in place of the then-obsolete heathendom. First attested in Francis Bacon, 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).