Religious belief set him off (6)
I believe the answer is:
theism
'religious belief' is the definition.
(theism is a kind of religious belief)
'set him off' is the wordplay.
'off' is an anagram indicator.
'set'+'him'='sethim'
'sethim' with letters rearranged gives 'THEISM'.
(Other definitions for theism that I've seen before include "of faith" , "Belief in existence of a God revealed to man" , "The doctrine or belief in the existence of a God or gods" , "Belief in a creator" , "Belief in a deity" .)