What if Islam never spread out of Arabia?

Hinduism and Buddhism aren't challenged by a strong monotheistic rival in the East...

Probably Nestorian Christianity will be more widespread than OTL. There were many converts in Persia at the time, and they would reach India and China too. While I'm not sure how sucessful they would be the Eastern Church will remain a greater voice within Christianity.

They did IOTL. And they had a generally more than decent relationship with Muslim authorities for a very long time. However, I concur that a stronger nestorian christianity is very likely ITTL.

Don't underestimate the Manichaeans!
 
The Schism hasn't even been finalized yet at this point. Heck Chaledonian Chrisianity was to busy dealing with other heresies like Arianism.

The Schism had not formally appeared, no, but there were definite differences between Western and Eastern Christendom, due to the vastly different cultural, political, and historical environments that they were operating in. The Schism was a slow-burning, gradual process that took centuries to go from "minor disputes, less serious than those found in the Empire itself" to "anathema and excommunication".
 
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