Holiness confused with Satan in two biblical books (13)
I believe the answer is:
thessalonians
'two biblical books' is the definition.
(I've seen this before)
'holiness confused with satan' is the wordplay.
'confused with' indicates an anagram.
'holiness' anagrammed with 'satan' can make 'THESSALONIANS'.
'in' is the link.
(Other definitions for thessalonians that I've seen before include "two biblical books" , "Old people" .)