Jim:
Oxford english dictionary: Infidel

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).