infidel (n.)

mid-15c., "adherent of a religion opposed to Christianity," from Middle French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (from PIE root *bheidh- "to trust, confide, persuade").

Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see kaffir), which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."