Laid down the law of French religious belief (7)
I believe the answer is:
decreed
'laid down the law' is the definition.
(I've seen this in another clue)
'of french religious belief' is the wordplay.
'of french' becomes 'de' ('of' in French).
'religious belief' becomes 'creed' (I've seen this in another clue).
'de'+'creed'='DECREED'
(Other definitions for decreed that I've seen before include "Officially ordered" , "Ordained" .)