Absolute truth (6)
I believe the answer is:
gospel
'absolute truth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gospel that I've seen before include "The teaching of Christ" , "Absolute truth" , "The teachings of Christ in a Book of the New Testament" , "Unquestionable truth - church doctrine" , "God's honest truth" .)