If we’re going to talk about the triumph of monotheism over polytheism, we’ll need to touch on a number of factors:
1. The reliance of (many, but not all) polytheistic faiths on state patronage and top-down ritual.
2. The importance of identity in the expression of faith as a form of communal well-being.
3. Sheer sociopolitical inertia.
State Patronage
When analyzing pre-Abrahamic polytheistic belief, an important factor is the role of the state in the maintenance of ritual and theopolitical infrastructure. Roman faith, for better or worse, was functionally tied directly to the state in every possible manner. To neglect one’s religious duties as a member of the government apparatus was decried as atheism. To exhibit atheism could bring on divine wrath, and thus bring harm to the Roman state – this was seen as a very real form of treason, and was one of the primary crimes early Christians were charged with. The Romans took their faith very seriously, and the concept of 'separation of church and state' was alien to them - the state was the church, the church was the state.
The maintenance of rituals, sacrifices, and the various temples that made up Roman religious life were an important part of the
res publica, and these institutions were completely reliant on the state to remain functional. Unlike Judaism, Christianity, or Islam, these institutions did not encourage (individual) literacy or allowed the existing practitioners to continue to exist even if local state authorities actively opposed them. Thus, when crippled, there were rarely individuals from the lower classes who would rise to replace the loss of institutional theological knowledge that was maintained in the temples and priesthoods.
Let’s say you’re a mid 4th century Roman – you make offerings at your local temple to the
daimones, the local god, and your ancestors. But you’ve also recently been going to something called ‘mass’ at a new Christian church. They even give out bread! This is fine with you, and you see nothing theologically at-odds with this practice, as people adopt new gods all the time. As you grow older, you notice the local temple is falling into disrepair, the rituals and sacrifices less impressive, and both of the head priests have died with no one to replace them. Meanwhile, the local church has grown in its monumentality. What once was a small and humble structure in your youth has become a much more impressive building. The priests are very well-funded, and the servants of this Jewish God can be found on many streets fulfilling the theological needs that the old temple used to provide. You still make offerings to both, but after you die, your son does not and frequents only the Christian church when in public. To make matters worse, the recently-issued Edict of Thessalonica has shut down the old temple in its entirety. The old faith will persist, for a time, as a home religion and the beliefs of the
pagani, country bumpkins. But nothing more.
Hinduism was lucky in this regard, in that its own examples of this state-sponsored ‘ritual trap’, Śrauta, had slowly but steadily declined even before direct contact was made with Christianity or Islam. By the time either faith interacted with Hinduism, it had already become more philosophical and inward-looking. This was something that was seemingly in-progress among the Greek world with its own turn towards what could be considered ‘theological philosophy’, but Christianity moved faster. The loss of institutional support is often the first hobbling blow to any religion, but for the Indo-European and Near-Eastern polytheistic faiths, it was the beginning of the end.
An excellent example of this in action was the death of pre-Abrahamic Egyptian religion. With the final collapse of the ancient line of Egyptian kingdoms and the land’s submersion to foreign rule for centuries, the massively-complex nature of Egyptian temples, rituals, and rites was unable to effectively continue. Temples lost funding and literally collapsed, priesthoods remained unmanned, and the rituals that underpinned rural and urban Egyptian life lost precedence to syncretised Greco-Roman deities paying lip service to local rituals, their Greek and Latin rulers rarely caring for the temples beyond when it suited them. That is not to say that it died completely, as we have evidence of its practice into late Antiquity, but it was functionally moribund without a Pharaoh and state-sanctioned priesthood to lead it. When Christianity rolled around, native Egyptians jumped ship fairly quickly, using it as a vehicle to assert themselves as separate from their Greco-Roman overlords. Which leads to….
Identity
An important mover of any faith, be it monotheistic, polytheistic, or anything else, is the relation of the in-group vs the out-group. Faiths, by and large, are convenient and useful vehicles to reinforce differences between groups of individuals, and the Abrahamic faiths are especially effective at this. Not only with individual action and appearance (dietary restrictions, attire restrictions, etc.) but also within an entire culture sphere. By the 5th Century, to be Roman was to be Christian, and to be Christian was to be Roman (usually). If you wanted to become Roman, you had to become Christian. If you were an invading Germanic tribe, and your leaders wanted to take part in Roman culture, you might as well worship the Roman God alongside your own deities. Or, even better, adopt an offshoot of the Christian faith as your own, allowing you to separate yourselves from your Roman subjects as you settle within their lands. Still ‘Roman’, but better than them. You could even worship both at the same time. Indeed, ‘dual-faith’ was probably the norm with many converts for centuries. Worship God at the local church, worship your ancestors at home, worship local nature spirits and your ancestor's Gods at the
hearg out of the way.
When a group’s identity is tied to their faith, it makes it far more difficult to convert them. To piggyback off of what
@Emperor-of-New-Zealand mentioned earlier, the Carolingian conversion of the Saxons necessitated conversion by the sword. Old Saxon belief is fascinating among European aboriginal faiths in that it was reinforced from the
bottom-up. Traditional ministry amongst the Saxons wasn’t getting anywhere – whenever an Anglo-Saxon or Frankish apostle had success with a Saxon noble, he would be deposed or killed by the
frilingi (yeomen free-farmers) and
lazzi (former slaves
). This seemingly-genuine expression of belief, and 200 years of on-and-off religious warfare, gives the impression that the Saxons were undergoing a genuine cultural and religious ethnogenesis of sorts. To be Saxon was to be Pagan, and to be Pagan was to be Saxon. Barring outside interference, this was not to change under penalty of death. Not to say that this wasn't the case with any other pre-Abrahamic faiths, but it's fascinating in that the Saxon peasantry seemed to be culturally universal in their opposition to conversion, rather than just certain subtribes.
The conversion of a plurality of Korea to Christianity gives another, more modern, example of the above. In the struggle for independence from both China and Japan, many in Korea took a third option and adopted Christianity. In the circles of early 20th century Korean intelligentsia, to be a Korean nationalist in the struggle against the Japanese Empire was to convert to Protestant Christianity. It was opposed to the
three-teachings (and later state-atheism) of China, and the Shinto-Buddhism of Japan, and presented a convenient method to reject both parties and reinforce their own culture. Furthermore, it was reinforced by direct contact with American missionaries, who brought practices that were considered ‘civilized’ and ‘modern’ in comparison to the perceived archaisms of localized Buddhist practice. Which leads me to…
3. Sheer Inertia
Piggybacking off of earlier points made within the thread, the decision to mantle
Romanitas and the Roman state to Christianity was the moment that Roman culture and sociopolitical influence would become synonymous with Christianity. As a result, to take part in Roman culture, one would (one way or another) be required to either convert to Christianity or accept its influence on their own society. To marry into a Roman family, and be accepted even as a foreigner over the lands you now rule, you had to convert to Christianity. To trade with Roman merchants, you were seriously pressured to convert to Christianity. As the centuries passed, Europeans associated the Roman Empire with Christendom, to the point that it was impossible to separate the two. Indeed, the Holy Roman Empire was perceived not just as a de-jure continuation of Rome itself, but indeed as a Holy
Christian Empire. To be Roman was to be Christian, and those that were outside of this sphere (Germanic, Slavic, Celtic, and Baltic tribes) would quickly re-orient and reinvent themselves to also mantle
Romanitas in their own ways.
Islam ended up with a similar inertia behind it – though the Umayyads allowed for conversion to Islam, it was heavily restricted, and non-Arab faithful were treated as second-class citizens in comparison to Arab Muslims themselves. That is not to say that they were treated overly-harshly, but rather that a dual society developed in which a small minority ruled over a large majority. Even with conversion to Islam, a mawali was still a mawali in comparison to the old Arab families. Thus, the Abbasid Revolution also created a new precedent – convert to Islam, and be treated as an equal in turn. This in turn led to Islam being (for a time) heavily associated with Arab-ness, and the successes of the Islamic world; thus the settlement of Arab tribes in the empty spaces where no tribes had been before, and the introduction of Copts and Arameans into the Arab identity in emulation. To be Muslim was to speak Arabic, and vice versa. Of course, the converted Persians would highly resent this, and would begin to reorient Islam under their own axis (indeed, the Abbasids gradually found themselves at the mercy of the Persian elite for running the everyday functions of the state). In time, to be Persian was to be Muslim (and better than those Roman and Mesopotamian barbarians), and the process would repeat itself with the intermingling of Turkic and Iranic cultures. Turco-Persian cultures would settle and clear land, and with that settlement came Islam and 'civilization'.
By the time full-global contact between continents has fully established itself, the dominance of monotheism over polytheism was nearly set in stone. To be ‘civilized’, and follow the trends of ‘civilized’ culture, was to adhere to Christianity or Islam, Rome and whatever flavor of Caliphate/Persia/Sultanate you like. The rise of East Asia threw a wrench in that theory, but it was functionally uncontested for centuries.