infidel

Short definition from Wiktionary

Adjective

1. 1
   Rejecting a specific religion.
2. 2
   Of, characteristic of, or relating to unbelievers or unbelief.

Noun

1. 1
   One who does not believe in a certain religion.
2. 2
   One who does not believe in a certain principle.
3. 3
   One with no religious beliefs.