infidel

An unbeliever with respect to a particular religion, especially Christianity or Islam.

One who has no religious beliefs.

One who doubts or rejects a particular doctrine, system, or principle.

Without faith: unbelieving; disbelieving; especially, rejecting the distinctive doctrines of a particular religion, while perhaps an adherent of some other religion.

Specifically Rejecting the Christian religion while accepting no other; not believing in the Bible or any divine revelation: used especially of persons belonging to Christian communities.