Belief in the Holy Ghost. He is manifest (6)
I believe the answer is:
theism
'belief' is the definition.
(theism is a kind of belief)
'in the holy ghost he is manifest' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THEISM' is hidden in the letters of 'holy ghost he is manifest'.
(Other definitions for theism that I've seen before include "The doctrine or belief in the existence of a God or gods" , "Belief in a creator" , "Belief in existence of a God revealed to man" , "Belief in a deity" , "of faith" .)