The doctrine that's a religious belief (6)
I believe the answer is:
theism
'a religious belief' is the definition.
(theism is a kind of religious belief)
'the doctrine' is the wordplay.
'doctrine' becomes 'ism' (suffix for disciplines or schools of thought).
'the'+'ism'='THEISM'
'that's' is the link.
(Other definitions for theism that I've seen before include "Belief in a creator" , "Belief in existence of a God revealed to man" , "The doctrine or belief in the existence of a God or gods" , "of faith" , "Belief in a deity" .)