Religious doubter (8)
I believe the answer is:
agnostic
'religious doubter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agnostic that I've seen before include "Doubtful, non-committal" , "He finds it difficult to believe" , "No firm believer" , "Believing that nothing can be known about the existence of God" , "Coasting away with non-believer" .)