My idea was that Christianity slowly filters outwards, moving from Canaan north into Phoenicia, east into Syria and south into Egypt. From Phoenicia it would spread to the greek polei and Carthage, diffusing into local culture over many centuries. Since there is no state-backed Orthodoxy enforced across the Mediterranean world, Christianity is widely heterodox. It doesn't become a very widespread religion in Gaul until ~800 and basically coexists with local paganism. The religious situation would be more akin to India, with a degree of separation and conflict between paganism and Christianity, but neither side fully destroying the other, instead slowly exchanging ideas over the millennia until the two religions are hard to differentiate for outsiders.
Empires like centralized religions: easy to control, unifying the people and counter-balancing uppity nobles. So I figured that Persia would adopt TL's closest equivalent to an "Orthodox" christianity, with the power of investiture and a monopoly on spirituality being an aspect of kingship. Manicheanism and Judaism would be very appealing to the Central Asians in this case, being a more "liberal" equivalent to Ctesiphoni Christianity. A Jewish Afghanistan would be neat, I thin