Someone who doubts beliefs, claims, plans, etc that are accepted by others as true or appropriate, especially one who habitually does so.
Someone who is skeptical towards religion.
Skeptical.
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.