“Even as Poitiers was being fought, collections of sayings attributed to Muhammad were being compiled that, in due course, would come to constitute an entire corpus of law: Sunna. Any detail of Roman or Persian legislation, any fragment of Syrian or Mesopotamian custom, might be incorporated within. The only requirement was convincingly to represent it as having been spoken by the prophet – for anything spoken by Muhammad could be assumed to have the stamp of divine approval. Here, then, for Christians was a fateful challenge. Their time-honored conviction that the true law of God was to be found written on the heart could not have been more decisively repudiated. No longer was it the prerogative of Jews alone to believe in a great corpus of divine legislation that touched upon every facet of human existence, and prescribed in exacting detail how God desired men and women to live. The Talmud, an immense body of law compiled by Jewish scholars – rabbis – in the centuries prior to the Arab conquest of the Near East, had never threatened the inheritance of Paul’s teachings as the Sunna did. Muslims were not a beleaguered minority, prey to the bullying of Christian emperors and kings. They had conquered a vast and wealthy empire, and aspired to conquer yet more. Had Francia gone the way of Africa, and been lost for good to Christian rule, then the Franks too would doubtless and eventually be brought to the Muslim understanding of God and his law. The fundamental assumptions that governed Latin Christendom would thereby have been radically and momentously transformed. Few, if any, who fought at Poitiers would have realized it, but at stake in the battle had been nothing less than the legacy of Saint Paul.”
Tom Holland Dominion: How the Christian Revolution Remade the World