No-account Roman historian's religious book (5)
I believe the answer is:
titus
'religious book' is the definition.
(Epistle of Paul to Titus in the New Testament )
'no-account roman historian's' is the wordplay.
'no' suggests deleting specific letters.
'account' becomes 'ac'.
'roman historian' becomes 'tacitus' (I've seen this before).
'tacitus' with 'ac' removed is 'TITUS'.
(Other definitions for titus that I've seen before include "A Roman emperor; a Pauline addressee" , "Groan maybe" , "Roman emperor - one of St Paul's correspondents" , "good book" , "Roman emperor from AD79" .)