INFIDEL. From the Lat. infidelis, meaning “unfaithful”, infidel came to denote one not among the faithful, or an unbeliever. Its earliest Eng. reference was to the Saracens, or Mohammedans, who embraced a religion opposed to Christianity. Mohammedans came to use an equivalent expression denoting all non-Mohammedans.
In the ecclesiastical language of Rome, infidelis came to mean unbelieving, and signified all those outside the church. Used particularly of missionary work in non-Christian lands, it was used synonymously with heathen. Missionaries or pastors whose work was “in part”—ibus infidelium—labored in predominantly unevangelized areas.
As used popularly, an infidel is a person who has knowledge of the Christian faith but avowedly rejects its claims to divine origin and authority. Usually a term of opprobrium.
International Standard Bible Encyclopedia (1915)