Masters of theological concept (8,3)
I believe the answer is:
original sin
'theological concept' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'masters of' is the wordplay.
'masters' becomes 'originals' ('original' can be a synonym of 'master').
'of' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'originals'+'in'='ORIGINAL SIN'
Can you help me to learn more?
(Other definitions for original sin that I've seen before include "Evil tendency that's common" , "Violations early in plot" , "Supposed innate tendency to evil" , "Man's innate offence" , "It ruins us all" .)