History is a mix of contingency and inevitability. It is not coincidental that human societies tended to aggregate in political size and grow in institutional scale and elaborate a more complex ideology. The early Ottoman Empire was quantitatively different than the Hittite Empire in all these things, and eventually, quantity has a quality of its own.
But there is an element of contingency as well. It is plausible that if Phillip of Macedon had not been assassinated the Persian Empire would have remained intact longer, with the Macedonians winning some victories to acquire extra territory and booty. The ascendency of Greek culture and language in the eastern Mediterranean may never have marginalized Aramaic as the lingua franca in the way that it did.
Shifting back to inevitability, I am now convinced that the quasi-animistic religion of the numina which was quite widespread in early Imperial Rome was not sustainable as the cult of a multi-ethnic polity. Something was going to emerge as a “new religion” which would serve as a stable religious matrix for the Roman system. There were various experiments and false starts. Vespasian brought back the cult of Isis as a personal devotee in the 1st century. In the early and late 3rd century there was an attempt to center the cult of Sol Invictus at the heart of the Roman pantheon. Diocletian, a traditionalist, set this aside during his reign.
More famously, Diocletian persecuted the Christians. Though there have been many academic debates on the magnitude of the Christian persecutions, it seems there is a consensus at that the last one under Diocletian was serious, though perhaps less eliminationist in intent than some chroniclers have depicted. The next step is well known. Constantine the Great first tolerated, and then promoted, Christianity as the religion of the Imperial court. Unlike Europe during the Westphalian period and down to the modern era, there was no “official religion” of a given polity, but a bundle of subsidies and favors from on high. Roughly, the period between 325 and 400 saw the decline of public elite paganism, the religions not Christianity, as the Roman elite converted to the new religion, and the Roman state shifted subsidies and supports from the old cults to the new.
The victory and centrality of religion with the general outlines of Christianity in broad features seems likely, though in its specific details there is a great deal of contingency. Early Christians exhibited social cohesion and were concentrated in cities. They could be instruments of state power through their parallel socio-political network across the Empire. The Christian religion presented a coherent and accessible ethos to the population and developed a sophisticated philosophy to appeal to the intelligentsia. It was functionally complex.
Peter Brown’s Through the Eye of a Needle: Wealth, the Fall of Rome, and the Making of Christianity in the West, 350-550 AD details the assimilation of Christianity into elite Roman society, and how the new religion was transformed by its new adherents, as well as how the religion transformed them. The point here though is that the relationship of Christianity to the Roman elite between 325 and 500 was dynamic and bidirectional. A Christian Church led by men of modest, though not abject, background, was transformed into an institution captured by the old nobility. Meanwhile, the Christian Church’s institutional independence was such that noblewomen could look to it as an ally in escaping the conventions of early and obligate motherhood.
The Final Pagan Generation and The Last Pagans of Rome outline how the elite transformed itself. Ideological Roman Catholics might argue that the rational genius of Christianity attracted the Roman nobility, while Marxist materialists view the conversion as one guided by pure self-interest. But human motives are more complex and self-justifying than that. Both books agree that some Roman nobles converted to Christianity out of sincerity, but both make it clear that a large number converted over time as it became first acceptable, then useful, and finally, necessary. Many scholars argue that the poet Ausonius converted to Christianity mostly because he saw that that was the direction of the world as it was. It was personally useful, but Ausonius seems to have never been particularly devout or plausible in his sincerity.
We know some of Ausonius’ thinking because his correspondence has been preserved. But what of the average Roman noble? It seems likely that most of them were conventionally religious, and initially conventionally pagan. The reaction of pagans and non-Christians to the religion are strikingly similar across time and place (e.g., Romans and Chinese both accused Christians of cannibalism due to the transubstantiation). But over time the new religion won more converts, either sincere or self-interested, and laggards would be noticed. One can imagine insincere converts who in their hearts remained pagan going through the motations. But over the decades the whole nobility converted to the new religion, and so over the decades, it took “root in their heart” in a more thorough way. The psychological change may have been gradual and imperceptible to them. Even if a deep Christian belief never takes root, a deep Christian identity may do so.
Obviously this changes in the generation of the children. Born as Christians, paganism for them is a shadowy religion that wafts down vaguely and in a blur from their familial heritage. For them their identity is undivided. Born Christians, in a world of Christians, their thoughts are unperturbed by heresy.
The social element in religious conversion is well known. The Mormon Church* obtains most converts not through its missionaries, but through friendship connections. A non-Mormon family sees and gets to know a functional Mormon family, and aspires to that sort of bourgeois happiness and contentment. One thing I have noticed among some people who are non-Mormons in a Mormon matrix (I grew up around many Mormons) is that they begin internalizing the views and beliefs of the majority without doing so consciously. The pagans of the Roman aristocracy may have undergone a nominal conversion, but surrounded by more and more devout Christians, it is inevitable that the contempt they may have toward the new religion turns into easy acceptance and even belief.
Christianity “took” in the West. The nominal Christians of the early generations became self-consciously pious in later generations, and later Europeans took their Christian identity for granted. Not all belief systems are like this. Some are quite thin. Their staying power is ephemeral. But the dynamics of the initial conversion phase is probably general.
* I know they don’t call themselves that anymore.