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.
One who believes dogmatically in something regardless of evidence or even conclusive proof that the thing is false or was staged; one who has true-believer syndrome.
Used other than figuratively or idiomatically: see true, believer.
A strict follower of a doctrine.