Even now in our time of decadent decline with a rise in atheism and cults, there is a material of religious belief that forms the stonework on which our civilization rests. Western Civilization is based on a Greco-Roman-Celtic-Christian system, but it would be foolish to not consider the basic laws, customs and norms of Christianity that permeate our world 2000 years after the time of Christ. Even the hippie ideals of the '60s are just basic Christian messages of peace, love, kindness tied up with the pill and free love. If one considers the world that Christianity entered in the 1st century AD, it is startling how successful it was and how quickly it took over a completely different Roman belief system. Monotheism conquered a polytheistic world in roughly 300 years, with very little in the way of writing or apostles doing work for the first 100 years after Christ's death and resurrection. How did it pull off such a feat? Christianity was helped not as much by its system of beliefs, but more so it's reaction to Roman society and rule, and the lack of voice for common citizens in the Roman Empire.
Before discussing how this worked for Christianity, I'll spend a bit of time discussing voice and exit. Stripped down to basics, in a declining system whether it is a corporation, a market, a government, etc. participants can either express a voice in the hopes that change or a correction is made or they can exit, leaving the system entirely. Exit can cause change as well if the system responds, which would then cause re-entry by participants. One could argue that during colonial times, citizens who had no voice in Europe and felt the system was corrupt or declining could exit for the colonies. As one looks around modern America, a great argument is that we are allowed a significant liberty of voice that is absolutely ineffective while we have no means of exit. Voting, participating, and posting countless things on the internet can all express displeasure, but the system does not respond. This isn't all that far from Imperial Rome.
Augustus had retained many of the forms of the Roman Republic, but had concentrated all of the power in his hands. He had done this by retaining his army, rewarding it, and ceremonially proclaiming to defend that which Rome stood for while gutting any counterweights to his execution of power. This had continued for decades after his reign as more foreign conquests happened, which pumped gold into the Roman economy and new slaves. Rome really was a war, slavery + conquest economy. Emperors after August rarely had his skill level as the emperors from 37AD-81 AD were not at his level nor at the level of the "Five Good Emperors" known as the Antonines. That 37-81 AD period was also the early period of the Church. Even under the Antonines, the citizens did not have a voice as rule was by Imperial decree and administration. With all of the repulican forms of checks and balances useless, voting by the citizens had no impact. Courts had already become corrupted with a loss of faith in the Roman rule of law. There were no large scale charitable societies or organizations as Roman society was at its peak, enjoying the moment of being the ruler of the known world. Rome's coffers were emptied to pay for luxury items from the East, which they had newly come into contact with after the conquests of the 1st century AD. Rome had mvoed far from the farmer-citizen-soldier foundations, and was moving into the decadent Empire that only had one way to go: down.
Christianity in its early stages was a bit of a protest movement of the Jewish faith, a reformation if you will. An entirely separate post would be a monologue (or personal theory) on what the early Church was in relation to the Jewish faith as a sect or splinter cell as well as the historicity of Jesus. Christianity had a great message that had an apocalyptic foundation. Before the world ends, let us be nice, forgive, love your neighbor, and go back to the roots of Jewish beliefs as time is short, we have one life and judgment is coming. That is a great message, but it was also much more restrictive compared to the polytheistic god system, ancestor worship, and libertine festivals of ancient Rome. Roman festivals turned into week long orgies, and Christianity was far from that. Would that really appeal to people caught on the down slope of the Empire? I do not believe so for one moment. People are people, and the same vices that plagued the Romans plagued other societies that followed, including today. While they were similar to Jews, they were different. They also would not allow statues of Roman gods in their temples, which the Jews did to appease Romans. Keep in mind that the Jews had just fought a horrible war vs. the Romans and lost, creating the diaspora as well as inner reflection on their faith after the fall of Jerusalem (2nd half of 1st century AD). It couldn't just be the belief system, especially when the belief system was a splinter of a religion that just fought and lost to the Romans.
Early Christianity had its own laws and rules, forbid Christians from serving in the Empire, provided food + simple goods to the poor, disabled and neglected, and basically attempted to form a separate society from Roman rule. The bishoprics of early Christianity settled disputes as well as gave legitimacy to unions and acts. The loyalty was to the Christian community, and the bishop returned this loyalty with behavior + beliefs that were condoned by the community. These were communities within the Empire. With the low hurdle for sustainability being food and shelter, to brign the recruits in the Church had to offer an alternative to Roman society. The Romans captured, tortured and forced Christians to renounce their faith, which happened often, but the renouncers were welcomed back with open arms by the Christian community. The leaders of early Christianity realized that numbers mattered. Voice mattered as well.
What also mattered was the ability of the common folks to have a voice in the Christian community. Early Christianity welcomed in all walks of life upon public confession, and it reached out to provide compassion and care for the very folks that the Empire did not pay any attention to for decades if not centuries. Freed slaves were allowed equal standing in Rome in direct contrast to Roman laws. People were denied a voice in Roman society and government, and the early Church communities provided an exit from Roman society. The early Church also provided a souce for voicing how the community should operate. it even gave people the chance to become martyrs for the faith, elevating their sacrifice and worth to that of the man they prayed to: Jesus. That is a level of expression that no Roman cultural system or event would allow. All socieites should provide the basic of food and security, and when outcast Romans saw that the early Christians could do so and allow them influence and a voice, they exited Roman society to enter the early Christian enclaves.
If one looks at the map for early Christian hot spots in the 2nd and 3rd centuries, there are many in what we call Turkey, Israel, Syria, Jordan and Egypt. Egypt was nearly 50% Christian by 300 AD. These pieces of the Empire were later conquests. Their attachment to Rome was not as strong as the Italy focused elements. There was a decent community in that period in Rome where finding outcasts and disaffected citizens would have been easy pickings. Focusing on those eastern communities, they had Roman rule imposed on them. Besides anger towards their conquerors, there was probably a problem of legitimacy of Roman rule in those areas. Greece, which included the coast of Asia Minor, had been highly developed prior to Roman conquest, and chafed at the enslavement of its people. Christianity offered an exit and challenge to Roman rule. This had to have been extremely appealing to eastern Roman ctiziens angry at the Roman governors appointed by Roman emperors who ruled strictly. These governors were like American appointed consuls in defeated enemy lands. Consider the varied success of American consuls the last 150 years, and then think back to Roman times. I doubt the Romans were much more successful.
Those conquered peoples would be itching for an out. The subjects in the east would be looking for an out of a power who was overextended and declining. The values of ancient Rome were a more martial or warrior centric belief system with little pity on the downtrodden. This would extend to conquered peoples in the east. Those values can be trumpeted successfully in times of expansion, victory and growth. In times of decay and decline, they lose their power. Christianity was a polar opposite with their beliefs, and could prey upon that decline. Christianity could offer a different belief system combined with a community that was outside the Roman system as a fresh new approach in amidst the decline of the Empire. It was not a triumph of peace, love and forgiveness. It was a triumph of creating a community that provided opportunities denied ordinary people in the Roman system that carried a friendly message and belief system contradictory to that of the declining Roman power.