Agreement opposing religious document (8)
I believe the answer is:
contract
'agreement' is the definition.
(I know that agreement can be written as contract)
'opposing religious document' is the wordplay.
'opposing' becomes 'con' (I've seen this in another clue).
'religious document' becomes 'tract' (I can't explain this - if you can you should give a lot more credence to this answer).
'con'+'tract'='CONTRACT'
Can you help me to learn more?
(Other definitions for contract that I've seen before include "Arrangement to commit murder" , "Covenant" , "Shrink from the business agreement" , "Become smaller" , "bargain" .)