Without the prestige Christianity had in Arabia, Islam wouldn’t exist- and for that to happen it needs to be the state religion of the eastern Roman Empire. I’m very skeptical of Greco Roman religion surviving in the western government when the east is as solidly Christian as OTL. Perhaps all you have is the Germans not becoming Christian and you get the same result, but Greco Roman paganism, with its intimate ties to the state could not survive when the state itself becomes Christian. When this does happen, you’d probably get the Caliphate trying to support Christian revolts among whatever Christian communities remained in the west and convert them to Islam