infidel /ˈɪnfɪd(ɛ)l / derogatory mainly archaic

▸ noun a person who has no religion or whose religion is not that of the majority: a crusade against infidels and heretics (the infidel as plural noun) they wanted to secure the Holy Places from the infidel.

▸ adjective adhering to a religion other than that of the majority: the infidel foe. – ORIGIN late 15th century: from French infidèle or Latin infidelis, from in- ‘not’ + fidelis ‘faithful’ (from fides ‘faith’, related to fidere ‘to trust’). The word originally denoted a person of a religion other than one’s own, specifically a Muslim (to a Christian), a Christian (to a Muslim), or a Gentile (to a Jew).