Religious book, old book, one I had translated (7)
I believe the answer is:
obadiah
'religious book' is the definition.
'obadiah' can be an answer for 'book' (I've seen this before). I'm not certain of the 'religious' bit.
'old book one i had translated' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'book' becomes 'b' (abbreviation for book).
'one' becomes 'a' (a thing is one thing).
'translated' indicates anagramming the letters.
'i'+'had'='ihad'
'ihad' with letters rearranged gives 'diah'.
'o'+'b'+'a'+'diah'='OBADIAH'
Can you help me to learn more?
(Other definitions for obadiah that I've seen before include "OT prophet" , "Old Testament book and prophet" , "Hebrew prophet" , "Single-chapter OT book" , "piece from Hebrew Bible" .)