The startling growth of Christianity was not just numerical, but geographical and social as well. It has long been noted that Christianity quickly gained converts in the cities of the empire. In fact, it appears that cities gained converts more quickly than rural areas, and port cities had a Christian presence more quickly than inland cities. Stark found that 64% of port cities had a church by the year 100, and that number had grown to 86% by the year 180.
The evangelization of the Roman Empire is one of the remarkable chapters in the history of the church. Behind the story of Christianity’s transformation from an overlooked and misunderstood sect to the official religion of the Empire stands an important question: why did Christianity gain such prominence in the Roman Empire? It is inaccurate and simplistic to point to Constantine’s conversion and the Edict of Milan as the primary answer to this question. Even before Constantine came to power, Christianity had grown significantly by the end of the third century. When Constantine adopted Christianity, he was not favoring an unknown religion, rather he was adopting a religion which had already gained traction in many parts of the empire amongst all social and economic classes. 
So, what allowed Christianity to flourish in the Roman Empire during the second and third centuries? It can be tempting to see this growth as inevitable. Modern observers can adopt the idea that Rome received the gospel so hungrily because it was somehow uniquely open to Christianity in a way that the modern Western world is not today. The historical record, however, tells a different story. Christianity was out of step with Roman culture in significant ways, but the early church experienced remarkable growth despite that distinctiveness.
This post will examine early Christian expansion, showing how extensive Christian influence was in the Roman Empire. A separate post will then consider Christian distinctiveness, demonstrating that the church grew despite tremendous cultural obstacles.
It is notoriously difficult to determine exact numbers when speaking about the ancient world. Not only is there a lack of documentary evidence and statistics, but scholars are divided on the reliability of the numbers that do exist. It is possible, however, to establish some basic parameters from which a general picture can emerge. The Book of Acts makes it clear that there were at least several thousand Christians by the middle of the first century (a remarkable growth considering the small number of disciples found in Acts 1:14-15). Nevertheless, the total number of believers at the end of the first century was probably still relatively small. Rodney Stark estimates that the total number of Christians in the year 100 was probably around 7,530 – or 0.0126% of the total population of the Roman Empire at the time. Drawing on his training as a sociologist, Stark has suggested a general growth rate of 3.42% per year, or 40% per decade, in the early church. While the Christian population of the Empire was a very modest number at the end of the first century, it had grown to encompass more than 10% of the population, or 6,299,832 Christians, by the year 300.
 Many helpful scholarly works have been written on the subject. For examples, see W.H.C. Frend, The Rise of Christianity, (Philadelphia, PA: Fortress Press, 1984). A more recent work (which shares a title with Frend’s classic study) is Rodney Stark, 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: HarperCollins Publishers, 1996). The classic study of evangelism in this period is Michael Green, Evangelism in the Early Church, (Grand Rapids, MI: Eerdmans Publishing, 1970). Another valuable resource is Eckhard J. Schnabel, Early Christian Mission Vol. 1 & 2, (Downers Grove, IL: InterVarsity Press, 2004).
 The details of this numerical and geographical growth will be discussed later in this paper.
 See the discussion in Robert M. Grant, Early Christianity and Society, (New York: Harper & Row Publishers, 1977), 1-13.
 Stark, “The Rise of Christianity,” 7.
 While this rate of growth might sound high initially, Stark suggests that it is not. “If we cut the rate of growth to 30 percent a decade, by the year 300 there would have been only 917,334 Christians – a figure far below what anyone would accept. On the other hand, if we increase the growth rate to 50 percent a decade, then there would have been 37,876,752 Christians in the year 300 – or more than twice von Hertling’s maximum estimate. Hence 40 percent per decade (or 3.42 percent per year) seems the most plausible estimate of the rate at which Christianity actually grew during the first several centuries.” Stark, “The Rise of Christianity,” 6.