Religious belief is shown in the mass (6)
I believe the answer is:
theism
'religious belief' is the definition.
(I know that theism is a type of religious belief)
'is shown in the mass' is the wordplay.
'shown in' means one lot of letters goes inside another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'the'+'m'='them'
'is' going inside 'them' is 'THEISM'.
(Other definitions for theism that I've seen before include "Belief in existence of a God revealed to man" , "Belief in a creator" , "The doctrine or belief in the existence of a God or gods" , "Belief in a deity" , "of faith" .)