Religious belief is dividing those people (6)
I believe the answer is:
theism
'religious belief' is the definition.
(theism is a kind of religious belief)
'is dividing those people' is the wordplay.
'dividing' means one lot of letters goes inside another.
'those people' becomes 'them' (I've seen this before).
'is' put into 'them' is 'THEISM'.
(Other definitions for theism that I've seen before include "Belief in a deity" , "Belief in a creator" , "of faith" , "The doctrine or belief in the existence of a God or gods" , "Belief in existence of a God revealed to man" .)