The original schism between Orthodox and Catholic was that they didn't recognize the Bishop of Rome as the leader of Christians.
From Junayd Shah of Persa
To the Emir of the Nile
The Shah has noted your expansion across Arabia. There is only one such purpose for such a move across the empty desert: to cut off his holy armies from the cities of Mecca and Medina. Ths is already a great insult, but know this: if you claim Holy Mecca as your own, your effrontery will be sufficient cause to unleash his Qizilbash fanatics upon you.
i.e. because of my religious orthodoxy, if a not Shi'a state claims Mecca, I'm petitioning to have a casus belli.