in·fi·del 
(ĭnfĭ-dəl, -dĕl′)
(ĭnfĭ-dəl, -dĕl′)n.
1. Often Offensive An unbeliever with respect to a particular religion, especially Christianity or Islam.
2. One who has no religious beliefs.
3. One who doubts or rejects a particular doctrine, system, or principle.
[Middle English infidele, from Old French, from Latin īnfidēlis, disloyal : in-, not; see IN-1 + fidēlis, faithful (from fidēs, faith; see bheidh- in the Appendix of Indo-European roots).]