Gospel? It's unopened book of the Bible (4)
I believe the answer is:
ruth
'book of the bible' is the definition.
'gospel? it's unopened' is the wordplay.
'gospel?' becomes 'truth' (I've seen this before).
'it's unopened' means to remove the first letter (without opening letter).
'truth' with its first letter taken off is 'RUTH'.
(Other definitions for ruth that I've seen before include "-- Rendell, novelist" , "Babe -, US baseball star" , "Girl's name; pity (archaic)" , "Lady" , "distress once?" .)