What would be the most likely time for a Persian Empire (Mesopotamia + Persian plateau) with a Christian (Nestorian?) ruler class to emerge? From what I understand, in Sassanian Persia, Christianity was beginning to spread so that almost all of Mesopotamia (where Ctesiphon itself, seat of the Sassanian Kings and their court, was located) was solidly christian.
Assuming a PoD that prevents Islam from existing or expanding beyond Arabia, would the Sassanian Kings be compeled to convert as Constantine did in the Eastern Roman Empire?