Scenario: The Arabic invasions of Carolingian France and Byzantium are successful, and from their they are also perfectly positioned to conquer Italy. By this point they have secured total control of Mediterranean and the two major Christian churches(Roman Catholic and Byzantine Orthodox I mean).
Now this doesn't avert the Christianity of Russia, Germany and Britain, but it does bring Southern Europe under the rule of an empire that transformed most of it's conquest- Iberia the obvious example- into flourishing civilizational centers.
Am I being overly optimistic, or does this hold the potential to replicate the thriving of Al-Andalus in the rest of Southern Europe- basically skipping the societal retardation of the dark ages? And what path would Northern European Christianity take unshackled from the influence and control of the Byzantine and Roman church establishments?