They were written by devout men of letters (8)
I believe the answer is:
epistles
'letters' is the definition.
(I know this)
'they were written by devout men' is the wordplay.
I cannot quite see how this works, but
'devout' could be 'pi' and 'pi' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for epistles that I've seen before include "Long communications" , "Biblical letters" , "Letters, missives" , "It sleeps among letters of the Apostles" .)