Really popular bible book bolstering theist's conclusion (2,5)
I believe the answer is:
in truth
'really' is the definition.
(synonyms)
'popular bible book bolstering theist's conclusion' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found within the answer.
'book' could be 'ruth' (book of the Bible) and 'ruth' is present in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for in truth that I've seen before include "As a matter of fact" , "Actually" , "To be honest" , "Really" , "Honestly" .)