"lack of religion, contempt of religion, impiety," 1590s, from French irréligion (16c.) or directly from Late Latin irreligionem (nominative irreligio) "irreligion, impiety," from assimiliated form of in- "not" (see in- (1)) + religio (see religion).
It is come, I know not how, to be taken for granted, by many Persons, that Christianity is not so much as a Subject of Inquiry ; but that it is, now at length, discovered to be fictitious. And accordingly they treat it, as if, in the present Age, this were an agreed Point, among all People of Discernment; and nothing remained, but to set it up as a principal Subject of Mirth and Ridicule, as it were by Way of Reprisals, for its having so long interrupted the pleasures of the World. [Joseph Butler, introduction to "The Analogy of Religion," 1740]