Before Sunday, no Pope destroyed books of rivals (9)
I believe the answer is:
opponents
'rivals' is the definition.
(opponent is a kind of rival)
'before sunday no pope destroyed books' is the wordplay.
'before' says to put letters next to each other.
'sunday' becomes 's'.
'destroyed' indicates an anagram.
'books' becomes 'nt' (New Testament).
'no'+'pope'='nopope'
'nopope' anagrammed gives 'oppone'.
'oppone'+'nt'='opponent'
's' put after 'opponent' is 'OPPONENTS'.
'of' acts as a link.
(Other definitions for opponents that I've seen before include "The other side" , "matches" .)