Belief in Supreme Being rejected by many atheists initially (5)
I believe the answer is:
dogma
'belief' is the definition.
(dogma is a kind of belief)
'supreme being rejected by many atheists initially' is the wordplay.
'supreme being' becomes 'god' (synonyms).
'rejected' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'initially' indicates taking the first letters.
The initial letters of 'many atheists' is 'ma'.
'god' written backwards gives 'dog'.
'dog'+'ma'='DOGMA'
'in' is the link.
(Other definitions for dogma that I've seen before include "Incontrovertible set of principles" , "Opinion viewed as truth?" , "Code of belief" , "Religious doctrine proclaimed as true without proof" , "Firm doctrine" .)