I'm not a scholar, but I am auditing courses in the subjects of Early Christology, Patristic Theology, Textual Criticism of the Bible, Biblical Hermeneutics, the History of the Early Church, and the History of the New Testament at Princeton Theological Seminary. I also audited courses in the same subjects at Union Theological Seminary.
I think there were several factors that predisposed Christianity1 to rapid, widespread growth, beginning with a scattered pattern..
My Understanding of the Factors that Led to the Rapid Rise of Christianity:
It began near the nexus of 3 continents: Europe, Asia, and Africa.
It began during the Roman Empire, while the Empire was still relatively strong and robust, and controlled much of Europe and the Mediterranean, including North Africa. It had access to an extensive network of roads, and it had never been so easy to travel great distances. In Paul's letter to the Romans, he says he is planning a trip to Spain. He traveled throughout Asia Minor, Palestine, Greece, Macedonia, etc. By our modern standards, this wouldn't be very impressive, but this was happening 2,000 years ago, when the closest thing to an airplane was a horse. People could travel more easily than ever before.
After the message of Christianity was altered to make it less Jewish in nature, it became fairly inclusive; but the earliest converts were Hellenized Jews. Thus, communities with a decent number of Hellenized Jews would be most willing to convert.
It was relatively unique in that it was a religion of belief rather than mere observance. This appealed to certain groups (again, Hellenized Jews), but it took some getting used to for pagans.
It was also relatively unique in that it made promises of eternal rewards for the people who had been denied rewards in life. This idea was new to pagans, but familiar to Jews.
It was evangelical, and some of the apostles were brilliant PR men. The first apostles were Jewish, and shunned pagans (because Jesus apparently told them to do so).
Shortly after Jesus died, the new movement began to alter his message. This process has continued to this day, although at a much less drastic rate after the first couple of centuries. This is a huge benefit because it allowed the early church to become whatever its congregants needed it to be. This is when the scattered "shotgun pattern" begins to seep outwards and saturate the map.
- For example, the religion is based on a devout Jewish itinerant preacher who lived under Roman occupation and wasn't very happy about it. The Romans brutally murdered him in an unimaginably horrible way, and as his followers attracted new converts, the Romans persecuted them. Yet the church leaders in the second, third, and fourth centuries were able to take this faith based on an outspoken critic (and victim) of Rome, and turn it into the official religion of the Roman Empire. Later, beginning in the Middle Ages, this religion based on a Jewish preacher became the biggest contributor to the spread of virulent anti-Semitism throughout Europe. These are remarkable transformations, and they demonstrate just how adaptable Christianity was.
It became a religion of the book very early on in its history. This made it mobile. A single preacher could travel to communities he thought would be receptive, have someone make a copy of the text, and then move on to the next town.
How Are These Factors Unique to Christianity?
This one isn't that unique, but the other major religion which began nearby was Islam, and it has enjoyed a similar rise to prominence. Judaism also began in the same region, but it didn't grow much because it was inherently exclusive. Jews usually don't proselytize, they stand apart. They are the Chosen People, and although you can become a Jew, they certainly aren't going to force you to do so, and in most cases, they don't even ask you to join the team.
This is unique. The major non-Abrahamic religions (i.e., everything except Judaism, Christianity, and Islam) began in the Far East. Buddhism and Hinduism were born in the distant extremes of Southeast Asia, which had the effect of cutting them off from Europe and Africa. Christianity, on the other hand, started at the point where Africa, Asia, and Europe meet. It was incredibly easy for it to sprawl out in all directions.
Buddhism and Hinduism began in a relatively specific, localized area, and spread outward in an even manner from the central point. Christianity had to spread out in a number of different directions, and only certain people (mostly Hellenized Jews) were receptive at first.
This too is fairly unique, as far as I have read, but I can't go into more detail because I don't know enough about how the creeds and doctrines of Islam and Judaism changed in their early history. It is certain, however, that Judaism and Islam now treat their scriptures as inalterable, albeit open to interpretation according to less rigid believers. Christianity has often had a somewhat softer approach to its scriptures.
This is only unusual in the context of the ancient Mediterranean world. The "religions" of Greece and Rome were not about believing the right thing or living according to the laws of God. In fact, neither Greek nor Latin had a word equivalent to "religion". Their gods didn't care how you lived your life, or how you treated other people, or what you believed. They just wanted you to talk about how awesome they were, and on their festival days, you had to give them stuff so they wouldn't get angry and smite you. Judaism seemed bizarre to Roman pagans, because they didn't understand why people would think that gods cared an how you behaved or what you believed, as long as you gave the, nice stuff.
This was a huge deal. Virtually every other religion in the history of the western world was totally mute on the subject of an afterlife, or if there was an afterlife, it wasn't very appealing: a dark, shadowy place where you sat around forever doing nothing and feeling miserable. Judaism started out the same way, but by Jesus' time, the idea of something like heaven was popping up. Jesus seized on this idea and ran with it. He described the most amazing afterlife ever conceived, and he threw in an added bonus: if you're cool, you go to the best place ever and never leave it, plus, everyone who has ever been a jerk to you will burn forever in hell! This was a huge selling point.
Also fairly unique. Judaism kept to itself for the most part, Paganism was obligatory but no one cared whether or not you actually believed in the gods as long as you paid lip service to them. Christianity was the first religion (in the western world at least) that actively sought out new converts. This would have been something that Roman pagans had never experienced before.
Similar to #3.
Totally unheard of in the ancient Mediterranean world before Judaism, but taken to new heights by Christianity. Again, the pantheistic "religions" weren't interested in what you believed, did, or said, so long as you made offerings to the gods when you were supposed to do so. Another reason why pagans took some time to get comfortable with Christianity, while Hellenized Jews were already primed for it.
A More In-Depth Explanation:
While Jesus was alive, he stayed away from cities until literally the last week or so of his life. He seems to have done so deliberately, although the reason for this is the subject of some debate. The explanation that makes the most sense to me is that he preferred his own people: poor Jewish peasants in tiny villages and hamlets, devout but illiterate, living unpleasant lives and desperate for good news.
In many passages of the New Testament, he shows considerable scorn for Hellenized Jews, who sometimes seemed to be more comfortable around Romans than other Jews. He also specifically forbade his disciples from preaching to Gentiles (i.e., pagans). This limited the potential audience, and informed the future actions of the disciples.
His only recorded trip into a city was the beginning of the end for Jesus. At Passover, he went to Jerusalem, probably for the first time. The authorities knew that Passover, when the Jewish diaspora returned to the Holy City, swelling its population from a couple hundred thousand to over a million, was a powder keg. Insurrection had popped up on Pasover before, and the Romans and their Palestinian Jewish lackeys (i.e., the Temple priests, judges, and scribes) were on high alert. Jesus caused a disturbance in the Temple, which caught the attention of the powers that be, and he was arrested that night and killed the next morning.
His disciples seem to have remained in Jerusalem after this, rarely if ever leaving the city or the surrounding area, except perhaps to go to their hometowns every now and then. They didn't have much luck trying to convert the Jews of Jerusalem, so it was up to others to spread the word about Jesus.
Jesus probably died around the year 35 CE. Paul was writing his letters, epistles, and homilies only 20 years later. Unfortunately, he never actually met Jesus, and his letters tell us almost nothing about the living Jesus, whom Paul describes almost dismissively as "Jesus-in-the-flesh". If you go through all of the letters known or widely believed to be written by Paul, and write down everything he says about Jesus' life, you will be shocked to find that all the relevant passages will fit on a single index card.
Still, there are few people as important to the rise of Christianity as Paul. He was apparently quite charismatic, and people paid attention to him. He had previously been a Pharisee, or a devout, zealous, almost militant Jew, and had been an active participant (and according to his own testimony, a leader) in the persecution of Jesus followers in Jerusalem after Jesus' death.
After his famous conversion experience on the road to Damascus, he did a complete about face. Rather bizarrely, he now preached of the redundancy of Judaism, and urged his flock not to become Jews in order to follow Jesus. This didn't go over well with the disciples, who were well aware that Jesus was an extremely Jewish man, and had preached almost exclusively to Jews. Paul came into conflict with the disciples many times, and he wrote surprisingly hostile things about them in his letters. But despite the fact that he almost certainly turned Christianity into something totally different from what Jesus had in mind, he set the stage for its later success.
Paul was on especially bad terms with the new leader of the disciples, James, the Brother of Jesus. The primary cause of friction between the two men was Paul's utter disregard for what Jesus had said and done while he was alive. Paul writes in one of his letters that he learned nothing about Jesus from anyone who knew him, and all his knowledge of Jesus came from his conversion event on the road to Damascus. He was openly hostile towards James, and to the other disciples, and insisted on preaching his own version of Jesus' message, which he basically made up himself.
Because the disciples were hounding his every move, and because they focused on trying to convert Jews, Paul decided to focus exclusively on Gentiles (pagans). He began to say blasphemous things about Judaism, which only served to further enrage the disciples, who repeatedly summoned Paul to Jerusalem to answer for himself.
In the early Christian texts known as the "Pseudo-Clementines", there is a story about the last confrontation between Paul and James. Paul is forced to humble himself and repent for his blasphemous words. He agrees to take a ritual purification bath, but afterwards, James chastises him. Paul's loses his temper and physically assaults James. He throws Jesus' brother down the temple stairs, and a mob sees this and swarms Paul (people in Jerusalem appear to have admired James for his dedication to charity and poverty, although they may have thought him a little weird).
This story is probably an invention, but it serves the purpose of demonstrating how hostile the relationship between Paul and the men who actually knew Jesus had become. We do know that Paul really was called to Jerusalem on a number of occasions to be disciplined for his preaching.
Paul would go to a new town, set up a shop (he is believed to have been a leatherworker of some sort), and talk to his customers as he worked. After a few weeks, he would leave for the next town, but he would also check in with the churches he had already created, usually by writing a letter, but if he heard something that worried him, he would send a trusted assistant to set things straight. If that didn't work, he would go back himself and read his congregation the riot act. Eventually, his constant clashes with the disciples, and their attempts to go behind his back to dissuade the Pauline churches of Paul's bizarre form of Jesus following, led Paul to go somewhere far enough away from Jerusalem that the disciples would not be able to interfere with his self proclaimed mission. He went to Rome, and after that we hear nothing about him, except that he was apparently crucified some years after he arrived. But his letters lived on, as did his influence on the Christian faith.
Because of his problems with the disciples, Paul kept moving further and further away from Jerusalem. He seems to have preferred small cities and large towns, and his primary audiences were mostly pagans with the occasional Hellenized Jew. The disciples weren't going to let him off that easy, and after he left a town, they would send their own emissaries to contradict Paul's erroneous teachings and set the record straight.
After Paul wrote the earliest letter which is still extant today, it was another 15 to 20 years before anyone wrote anything about Jesus. Fortunately, this time someone wrote his life story. The first gospel to be written was the one we know as The Gospel According to Mark2, which most scholars date to 65-70 CE. Matthew and Luke were probably written between 85-95 CE. In fact, Matthew and Luke may have been writing at the same time as one another, and it is clear that both had access to the Gospel of Mark, but Matthew and Luke didn't know each other and neither had seen the other's gospel before writing his own. John was written much later; scholars usually date it to between 90-110 CE. Although none of these men ever met Jesus, and aside from Paul, they may not have met anyone who had met Jesus; John almost certainly didn't meet anyone who knew Jesus.
What we do know, or at least strongly suspect, about the authors of the gospels, is this: Mark may have been Jewish, but he wasn't well learned or educated. He gets some basic facts about Judaism wrong. His writing style is somewhat simple, repetitive, and plain. Matthew was either a Jew or someone who was very familiar with Judaism, and his gospel is the most Jewish gospel. Luke is almost certainly not a Jew, and in his gospel we begin to see traces of anti-Semitism. John was definitely not a Jew, and the Jews are slandered and demonized throughout the text. Taken together, this seems to suggest (rather strongly) that by the time the first gospel was being written, Judaism and the Jesus Movement were already well on the way to becoming separate religions. By the time the last gospel was written, Christians were becoming actively hostile to Jews.
This was probably because the proselytizes had finally realized that devout Jews just weren't interested in Jesus. I don't have the space to explain here why this was the case, but it had to do with the Jewish concept of the messiah. Jesus clearly didn't fit the description, and Jesus followers took the Jews disinterest as a sign of wickedness.
This is when Jesus' anti-Roman sentiments were erased from the gospels. The proto-Christians realized that Roman pagans were now more receptive to Christianity than anyone else, and so they made Jesus friendly towards Rome.
The scattered dots on the map began to spread more quickly now. Towns and cities with little or no Jewish population began to host churches. The Romans began to see the difference between Judaism and Christianity. Word spread.
Then came Emperor Constantine. He converted to Christianity (probably as much from political self interest as from genuine devotion), and although he didn't make Christianity the official religion of the Empire, he did abolish several laws which had persecuted Christians, confiscated their property, and made life miserable for them. A later Emperor reinstated the laws, but they were abolished permanently soon afterward, and Christianity finally became the religion of the Roman Empire.
Once the stigma attached to Christianity was removed by the conversion of Constantine, nothing stood in the way, and the religion spread like wildfire. The map now became a sea of Chritians. In the ensuing centuries, it would cross the seas and consume the Americas, large swaths of Africa, Australia, and parts of Eastern Asia.
Just How Fast Did Christianity Grow?
The sociologist Rodney Stark wrote a book called The Rise of Christianity: A Sociologist Reconsiders History, or How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries. He found that during the first three and a half centuries of its existence, Christianity grew at a steady rate of 40% per decade, or 3.42% per year3. This trend ran out of steam shortly after the death of Emperor Constantine, but only because there weren't enough people to continue the trend.
Constantine became Emperor in the year 306 CE. His conversion began in 312 CE, and he died in 337 CE; during his reign, the population of the Empire was roughly 60 million people. According to Stark, in 300 CE, the Christian population of the Empire was about 6.3 million, or roughly 10.5% of the total population. By 350 CE, only 13 years after Constantine's death, there were nearly 34 million Christians in the Empire, accounting for 56.5% of the total population.
The Rise of Christianity, Chapter I: Conversion and Christian Growth' Rodney Stark, p. 13
The 40% per decade trend ended around 350 CE, because there simply weren't enough people for the trend to continue - if it had, there would have been nearly 190 million Christians in the Empire by the year 400 CE, which would be more than three times the total population of the Empire. Suffice to say that by 400, or 450 at the latest, virtually everyone in the Empire was Christian.
Today, there are well over a billion Christians in the world, and no religion has ever been so influential in shaping the course of human events, for better or worse.
Stark points to a number of advantages that Christianity had over paganism to explain its growth:
While others fled cities, Christians stayed in urban areas during plague, ministering and caring for the sick.
Christian populations grew faster because of the prohibition of birth control, abortion and infanticide. Since infanticide tended to affect female newborn more frequently, early Christians had a more even sex ratio and therefore a higher percentage of childbearing women than pagans.
To the same effect: Women were valued higher and allowed to participate in worship leading to a high rate of female converts.
In a time of two epidemics (165 and 251) which killed up to a third of the whole population of the Roman Empire each time, the Christian message of redemption through sacrifice offered a more satisfactory explanation of why bad things happen to innocent people. Further, the tighter social cohesion and mutual help made them able to better cope with the disasters, leaving them with less casualties than the general population. This would also be attractive to outsiders, who would want to convert. Lastly, the epidemics left many non-Christians with a reduced number of interpersonal bonds, making the forming of new one both necessary and easier.
Christians did not fight against their persecutors by open violence or guerrilla warfare but willingly went to their martyrdom while praying for their captors, which added credibility to their evangelism.
Stark's basic thesis is that, ultimately, Christianity triumphed over paganism because it improved the quality of life of its adherents at that time.
1 In the beginning, people who followed Jesus weren't called Christians. In fact, we don't know if they were called anything in particular. During Jesus' lifetime, he and his followers were simply Jews. This was still the case for all of his followers for another couple of decades at the least, and most of them probably consider themselves exclusively Jewish as long as they lived. It was only in the second and third generations of Jesus followers that the slow but steady drift away from Judaism began. Some small sects of Jewish Jesus followers remained active until perhaps the fifth century or later. This situation cause problems for scholars, who like to have specific names for things. Therefore, the most popular term for the first followers of Jesus are now known as "The Jesus Movement". It bears repeating that these people were completely Jewish in belief, practice, and self identification. Furthermore, non-Jews also considered these people to be Jewish. It isn't clear when the Roman government began to notice the difference between regular Jews and Jesus Jews, but it is known that they had begun to single out Christians (who were by this time really Christians) by about a hundred years after Jesus died.
2 As far as modern scholarship has been able to determine, there is probably no reason to believe that the authors of the gospels were named Matthew, Mark, or Luke. There is a possibility that the author of some sections of John was actually named John, but the traditional claim that this was John the Beloved Disciple is not taken seriously by most impartial scholars. The earliest manuscripts of the gospels don't have any names attached, and there is no evidence that they were called by the names we know until a century or more after they were written. These names were attributed to them by the early leaders of the church, and some records remain of some of these leaders expressing their doubts about the issue of authorship. However, I will refer to the unknown authors of the gospels by the names associated with the gospels.
3 Interestingly, the Mormon church has grown at a rate of 43% per decade for the last century.