Starts to feel apostatic - it tests his religion (5)
I believe the answer is:
faith
'religion' is the definition.
(synonyms)
'starts to feel apostatic it tests his' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'feel apostatic it tests his' is 'FAITH'.
(Other definitions for faith that I've seen before include "religion" , "Confidence" , "Religious belief" , "Trust in someone" , "Total trust" .)