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)

the King James Version has this word twice: "What part hath he that believeth with an infidel?" (2Co 6:15); "If any provide not for his own, .... is worse than an infidel" (1Ti 5:8). In both passages the English Revised Version and the American Standard Revised Version have "unbeliever" in harmony with numerous other instances of the use of the Greek apistos. The word nowhere corresponds to the modern conception of an infidel, one who denies the existence of God, or repudiates the Christian faith; but always signifies one who has not become a believer in Christ. It was formerly so used in English, and some of the older versions have it in other passages, besides these two. It is not found in the Old Testament, but "infidelity" (incredulity) occurs in 2 Esdras 7:44 (114).