Teejay
Gone Fishin'
You completely avoided my point. Who or what could possibly unite the Arabs in time to exploit the temporary weakness of both empires after the Great War?
Making one religion into the official religion for the whole empire, it would likely be some form of Christianity (probably the Nestorian form). This process happened to the Germanic tribes which invaded the Western Roman Empire and the Mongols.
In this TL Persian civilization re-merges in a Nestorian Christian form, although absorbing a lot of Zoroastrian influence.