Rodney Stark in his book The Rise of Christianity looked at the situation from a sociological rather than a theological, philosophical, or political lens. His work isn’t perfect, as we will see, but it is what will make the bulk of my answer. Unfortunately, due to closures related to COVID-19, I can’t quote directly from this source as there is no ebook, and I can’t access a print copy. Instead, I use secondary sources who use him as a primary source and one of his other books, written for a less academic audience. It’s not my preference, but such is a sign of the times.

Stark’s most compelling assertion is conversion takes place not because of conviction about doctrinal truths, but because of close social networks. “Put more formally people: people tend to convert to religious groups when their social ties to members, outweigh their ties to outsiders who might oppose the conversion, and this often happens before the convert knows much about what the group believes.” (Stark, The Triumph of Christianity). This means that once people were converted, they worked to convert their friends and family and so conversion traveled from community to community.

Secondly, Stark states that people who are very religious are unlikely to convert to another religion. It was those who were not very religious who were likely to convert. However, those who do convert have some level of cultural continuity between the religion they are leaving and the one they are joining. There were two groups that could convert to Christianity, those who were Pagan and those who were Jews. Both had varying levels of cultural continuity.

During this time the Jews were “in Diaspora” that is they were a people without a land to call their own, dispersed rather than together in community. The Jews as a people had not chosen to spread out like this; instead, it was a result of the conquest policies of Ancient Babylon. The Babylonian culture was replaced with the Persians, then the Greeks, and then the Romans. Living in other cultures and yet worshiping and acting as their own culture demanded was difficult, and so many were not as observant as might be expected.

Additionally, Christianity had cultural continuity with Judaism. Jews who were converting did not have to give up their Old Testament background, or their monotheism as they would have had to do if they converted to paganism. Nor did they have to rigorously keep the old law or learn a new set of rules. They only had to add a messiah to their religious understanding. And even for that, they had an existing framework.

Scholars are divided on when the conversion of Jews stopped. Still, there is archeological evidence of Jewish and Christian communities within close proximity to each other in the 3rd and 4th Centuries in Rome and into the 7th century in Palestine. So it is reasonable to conclude that Jewish conversions could have happened until those times because of the factors mentioned above.

Cultural continuity also existed for Christian converts from paganism, though it looked a little different. Stark asserts that certain aspects of the Christian gospel would be familiar to pagans including, the immaculate conception, the crucifixion as a sacrifice for the attainment of sin, and the existence of miracles.

Third is Stark’s assertion that contrary to popular belief, religious movements --including Christianity are launched by the privileged, not the poor. There is some evidence of this in the letters that Ignatius, Bishop of Antioch, wrote around 110 CE when he was arrested and sentenced to the arena in Rome. He wrote to or mentioned those in high political standing, such as the wife of a procurator. His letter to the church in Rome implies that some people in the congregation may be able to get him out of his sentence, and he asked them not too. In the late 3rd century, 10% of the Senators of Rome could be identified as Christians. The power and influence of the privileged served to spread Christianity further as they moved throughout the empire in ways that the lower class would not have been able to.

Stark also asserts that Christianity spread quickly because of its appeal to women. In general, women convert to religions in higher numbers than men. But Christianity also was better for women than either Judaism or the paganism of the time. They tended to be able to marry older (rather than being child brides) and likely had some say in who they married. In Rome, at the time, there were an estimated 131 women to 100 men. This means that women who were more likely to convert were also a larger part of the population. And then they may have converted their husbands due to social networking and raised their children in the faith. They also tended to have more children due to Christian aversion to the Roman practices of exposure of unwanted infants and abortion.

Stark asserts that the growth rate of Christianity was also affected by the plagues in Rome. This seems less clear and less supported by other historians than some of his other points. His theory is that Christians had a reputation of caring for each other and the pagans in their sphere of influence when the plague hit. Many attributed miracles of survival to them, even if it was just that the sick were strengthened by getting food and water. Certainly, some Christians died, but according to Stark, a larger proportion survived, which increased the rate of growth. However, as Vaage notes,

“A fly in the ointment is the fact that no Christian writer on the subject— and there are not many who wrote on this subject— boasts of superior survival rates. We assume that they would have done so, if matters were as dramatic as Stark suggests. It would have been good publicity.” (221)

Another critique of Stark’s point that Christian mercy lead to the spread of the faith is that he characterized paganism to harshly and that there were expectations that Roman pagans would contribute to civic life and care for the poor.

Finally, the church spread rapidly in part because it spread in cities. Within 20 years, it was in urban environments, first port cities, and then others as travel in the Mediterranean was easier by ship. Growth in Hellenic cities happened faster than in Roman cities, possibly due to the role of women, as mentioned above.

Overall, Stark suggests that a growth rate of 3.4 percent a year is plausible based on data found in the increasing percentage of Christian gravestones and on contemporary religious movements for which there is good data available. Some scholars still urge caution with these statistics while admitting that Stark has raised interesting questions and looked at the rise of Christianity from a new perspective

“As Stark sketches possible scenarios for Christian growth based on mortality rates, he crunches the numbers impressively (1996, 89– 90, 91– 92). Eventually the numbers take on a life of their own, and assume a solidity they simply cannot have. We have to remember that, unlike usual statistical presentations, Stark’s discussion is not based on data that have been scientifically collected and analyzed. Rather, this is admittedly hypothetical material (Stark 1996, 89, 91), based on estimates rather than hard data, which have been put on the table in order to see if Stark’s assertions about Christian growth are plausible” (Vaggae, 228-229)

In summary, social factors that may have worked in favor of the rise of Christianity are social networking, cultural similarity to existing religions, appeal to women who made up a larger percentage of the population, and its appeal to those of privilege and especially the merchant class who would have helped it travel between cities. I don’t claim that social factors are the only factors, but perhaps there are others who are more suited to answer that.

Sources

Vaage Leif E. ed. Religious Rivalries in the Early Roman Empire and the Rise of Christianity, Wilfrid Laurier University Press, 2006.

Stark, Rodney. The Triumph of Christianity: How the Jesus Movement Became the Worlds Largest Religion. New York: Harper One, 2012.

(Mentioned) Stark, Rodney. The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries. New York: HarperOne, 1997. (or 1996 depending on the version)