It's religiously conclusive (4)

I believe the answer is:
amen
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for amen that I've seen before include "Put name to end of payer" , "Name for prayer's end" , "Last word of prayer" , "Final word in prayer" , "prayer's ending" .)
