Something that struck me about the rise and spread of Christianity throughout the Roman Empire and later on the Western World was how much it differed from the predominant religions in the region. The Roman religion, like the religions of many of their neighbors, was an ethnic one. Romans worshiped Roman gods (who they cribbed from the Greeks, of course); Germans worshiped Germanic gods, Egypt and Nubia had their own pantheon, the Phoenicians (including the Carthaginians) had their own gods, and so on. The Roman State paid tribute to Jupiter and his cronies, but they were open to most other religions – and in fact the Romans often adopted foreign gods into their pantheon, or at least modified their own conception classically Roman deities with ideas and concepts from other religions.
Judaism, of course, is where the difference comes from. The worship of these pagan deities didn’t require exclusivity – you could pay your tribute to Jupiter or Athena or Re, and you didn’t necessarily take issue with the fact that your neighbor worshiped a different god. Judaism was different. It said, no, there is only one God, and it’s our God. The others are false idols. And yet that’s as far as Judaism took it – it was still an Ethnic religion. Judaism was meant for the Israelites, and by the tenants of Judaism everyone else could follow whatever religion they wanted to (Unless they worshiped Baal, of course. It seems that lots of cultures took issue with Baal worship – the Romans painted the Carthaginians in a very negative light due to their worship of Baal. Whether this is because Baal worshipers actually practiced human sacrifice, or if that was Roman propaganda, or some combination of both, historians still argue about). Just this exclusivity gave the Romans enough problems that they persecuted Jews, despite the Roman Empire being, on the whole, relatively tolerant of anyone who could agree to the stipulation that yes, you can worship whatever god you want, but you must worship the State first.
Enter Christianity. It kept the idea of a single true God, to the exclusion of all other deities, from Judaism; but it ditched the ethnic link that most other religions in the region had until then. Like many other religions and cults, Christianity gained some popularity in Rome, but something about it – perhaps its proselytizing nature – led to it becoming more and more widespread, despite and perhaps because of its persecution, until it became the state religion of the Roman Empire. Mithraism, for example, was popular for a time; but it didn’t have the same staying power that Christianity seemed to.
But what if Christianity never took hold in Rome, at least not as an official state religion? What if like countless cults and beliefs before it, it spread through the Roman Empire before burning out and being replaced by something else? I would argue that this would have a HUGE impact on the latter history of Western Europe. Many of the same “barbarian” kingdoms who helped bring about the fall of the Roman Empire would later try to emulate it – sometimes before it even truly fell. Most of these kingdoms would convert to Christianity, often not because the subjects of said kingdoms actually believed in that religion, but because adopting the state religion of the late, great Roman Empire lent legitimacy to the ruler’s claim to the rest of Rome’s lands. It is from this time period that we have art depicting Christian events with a very Pagan twist to them, such as depictions of Jesus wielding a spear and - possibly - with a missing eye, borrowing heavy from Norse traditions and depictions of Odin.
Without Christianity, what would these successor kingdoms behave like? What would a kingdom like Charlemagne’s look like without a Pope? Would Europe have been far more religiously divided, and would Rome still hold such a prominent place in our culture, 1600 years later? I’m under no illusion that a strong state-sponsored religion is in any way necessary for an Empire to form and justify its conquests – but it certainly helps. More than that, Medieval Europe was a violent and war-torn place, even though religious differences were relatively minor. What would a Europe full of competing ethnic religions look like a thousand years later?