What would world history be like if Rome had not converted to Christianity?Having conquered and united Western Europe in the early middle ages, Charlemagne would have associated himself with the main religion of the fallen Roman Empire to gain prestige and legitimacy. In history that would have been Christianity.
In your alternative history, he might have adopted the religion of Rome at the point of its collapse. I don't claim to be an expert in these matters, but a swift look at Wikipedia, that would be a polytheistic belief system. That is, believe in many Gods such as Mars and Jupiter. Romans also adopted many religions of its conquered territories such as the Greek gods. There was also numerous religious cults that were being practiced in Rome such as Mitraism and Sol Invictus. The Romans also did not make a distinction between the religion authorities and the state, and office of priesthoods were held by members of the elite ruling class.
Assuming this system was adopted by Charlemagne, his successors would have perpetuated this religious system across Europe throughout the Middle Ages. Having said all this, even against this 'alternative' setting, I'm not sure whether Medieval European history might have changed significantly from 800 to 1095 (the year of the first Crusades). The names and banners may have changed, but human politics - greed, egoism, survival, security and market/trade expansion - would still be main driver of collective monarch-nation-state actions.
Coming to 1095, the year of the first Crusade, I don't think this event would have happened because Jerusalem is not significant to the Roman religion. And hence the western nations would not have felt the need to 'save the holy land'. However, a military response from the European monarchs might still have been formulated against the invading Muslim armies, perhaps at much later years, and under a different rallying banner.
Moving on to the Great Schism, the reformation, the counter reformation, rivalry between Catholic and Protestant nations in Western Europe, revolution and nation building in the 14th-19th century, I think similar forces would still be at work to 'free' the masses of ordinary people from monarchy-religious authority top-down control. In history these great human movements were sparked off by ordinary people getting fed up with corruption in authority, and perhaps also driven by the pressures of increasingly difficulty daily life, which has been under a constant pressure of monarchy driven war, famine and overpopulation. These forces would still have been around, and would be directed at the prevailing religious authorities (and ruling monarchs), in this case, the polytheistic Roman belief system.
As a round up, it seems to me that history might not have changed so much. Just the names and banners would have changed. This is assuming that the main driver of human actions would have been political, and the leaders at the time have used religion as a uniting point or a rallying point to push their cause.
You know? I really would like to know the answer to this very question. Something tells me it would be a much better world, because the Faith was never intended to be a political power, but a force of personal change and, well, salvation. One cannot read the 5th chapter of Matthew, for instance, without realizing that the transformation of Christianity from a way of life to a political system was at terrible odds with Jesus' intent. "The Kingdom of God" was NEVER on earth! Even in his face to face with the choice between death and "life", he told the powers of the world that "My kingdom is not of this world. If it were, my servants would fight to prevent my arrest by the Jewish leaders. But now my kingdom is from another place." (John 18:36) And yet, even today, the very inheritors of Constantine's "vision", the Dominionists and right-wing political leaders of the U.S., still try to fight to make his kingdom "of this world"!
An interesting aside I have just noticed. How often have you heard "law-and-order" types say things like: "Well, if so-and-so wasn't guilty, he wouldn't have been arrested and/or indicted"? (unless it's a cop, of course!)
Guess what? It's the same thing the Jewish leaders said about Jesus: "If he were not a criminal," they replied, "we would not have handed him over to you."
Who are the models for your justice, your mercy, your humility? God? Or man?
Mithraism and Manichaeism could have been the two alternatives that were more popular at one time than early Christianity, but Constantine wiped those cultures out of the empire upon threat of execution. Interestingly, both emanated from Persia and involved the Soma sacrament in some form, although the Romans may have substituted fortified wines and spirits for milk. The other main ingredient was cannabis. Islam came much later. It's interesting that Constantine's version of Christianity and Islam both came from empire-building generals who deployed the religion to inspire the soldiers. Mani had called himself "a disciple of Jesus" and many elements of his story ended up in Constantine's Christianity, and he was conducting a hippie-like peace movement that sought to blend all religions as one. There likely would have been no wars conducted on cannabis and opium had Christianity not become the state religion of Rome.
A much better place.
O.K. Rome would have lasted longer. It would have eventually unified Europe all the way to the Middle East.
Since the Romans did not value theory, chances are the world would not be as advanced technologically. Greek mathematics and philosophy made it to the main stream primarily by way of Christian scholars. Without the collapse of Rome it is unlikely the Greek philosophy would have made it to the middle east and there probably been no Islam. And even if Islam did arise the Romans would squash it flat in their patented heavy handed manner.
Mithraism was the main competitor to Christianity in the first centuries AD and remained a challenger up until the third century AD when Christianity triumphed.
Going by the following extract and what I've seen of human nature and human organisations I think the world could well be pretty much as it is now.
Because Mithra (or Mithras in Latin and Greek) denounced social injustice and preached the brotherhood of man, Mithraism found widespread acceptance among slaves, the poor and the exploited. Many of its downtrodden devotees were willing to die for their faith.