One who does not believe in a certain religion.
One with no religious beliefs.
One who does not believe in a certain principle.
Rejecting a specific religion.
Of, characteristic of, or relating to unbelievers or unbelief.
A disbeliever, a denier: someone who rejects or disbelieves in God as per Islam (Arabic: الله Allāh) or the tenets of Islam; or more broadly any non-Muslim.