Religious sort to differ about holy books (6)
I believe the answer is:
votary
'religious sort' is the definition.
(religious person)
'differ about holy books' is the wordplay.
'differ' becomes 'vary' (varying is a kind of differing).
'about' indicates putting letters inside.
'holy books' becomes 'OT' (Old Testament).
'vary' placed around 'ot' is 'VOTARY'.
'to' is the link.
(Other definitions for votary that I've seen before include "Devoted follower" , "One by promise bound" , "Monk or nun who has taken vows" , "One dedicated to cause, religion" , "Devoted adherent of a religion" .)