The Christian Roman Empire prospered between 320 to 370 CE? But the empire wasn't fully Christianized until Theodosius the Great, who started his reign in 379. Even then, Christianity was in its early days.

My point was that apocalyptic, countercultural Judaism and paganism don't go so well together. So while the empire might have flourished economically and militarily, it would have been a cultural abomination, which again is what we see in the medieval West, compared to the more united early Muslim world, which went back to pure monotheistic Judaism.

The barbarians resented the empire as much as they admired it, and they warred with it as much as they fought for it. They wanted to be part of the empire in the way that the mafia wants to be part of your business, as a protection racket.

There are a thousand conflicting reconstructions of the historical Jesus, which was the point of Schweitzer's The Quest for the Historical Jesus. The available evidence is too weak and ambiguous for us to know what the historical Jesus was like or even if there was an historical Jesus. Was he an apocalyptic preacher? Was he an itinerant wise man, like in Q? Was he a mystic or a healer? Was he a political agitator? Was he a proto-Gnostic? You can emphasize parts of the evidence to prove whatever you want.