Believing that nothing can be known about the existence of God (8)
I believe the answer is:
agnostic
'believing that nothing can be known about the existence of god' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for agnostic that I've seen before include "Coasting away with non-believer" , "Uncertain type" , "Religiously non-commital" , "Sceptic - unbelieving" , "I doubt" .)