I've always been fascinated with the Christian Period of Rome. People don't realize how entrenched Christianity was in Roman society prior to Julian I's religious reforms, not just among the elites, but among ordinary people. If Julian hadn't attempted to root out Christianity, or even if he'd been less successful at it, it seems likely the Church could establish a more permanent presence in Western Europe. And if the Church can stay on friendly terms with Rome, it seems decidedly unlikely that the Sassanian would become very tolerant of them, let alone convert themselves. With a hostile Persian Empire in the way, what would be the fate of Christian missionary activity in Central Asia and the Subcontinent? Would China ever have a Christian Emperor? Also, what do a Christian Europe and a European Christianity actually look like? It honesty seems like a pretty feasible scenario, but damn if it isn't difficult to imagine.