A Growing Church
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.
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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. These figures are, of course, estimates, but they reflect the general scholarly consensus regarding the incredible growth of Christianity in its first few centuries. Not without reason was Tertullian able to taunt his opponents (with a touch of hyperbole) saying, “We are but of yesterday, and we have filled every place among you—cities, islands, fortresses, towns, market-places, the very camp, tribes, companies, palace, senate, forum—we have left nothing to you but your gods.”
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. While growth in inland cities was slower, it was still significant, with 24% of inland cities having a church by 100 and 65% having a church by the year 180.
Cities were strongholds of Christian influence, but the rural areas were not overlooked. Though exact numbers can be hard to find, recent research has shown that rural areas in Rome had far more of a Christian presence than what has traditionally been thought. By the end of the second century, Christianity had grown both numerically and geographically.
Christian growth was even more remarkable in that it encompassed all social classes. When the Emperor Trajan instructed the Roman governors to persecute the early Christians, Pliny the Younger (A.D. 61-113), the Roman governor of Bithynia, wrote to the Emperor:
“For the matter seemed to me well worth referring to you—especially considering the numbers endangered. Persons of all ranks and ages, of both sexes, are, and will be, involved in the prosecution. For this contagious superstition is not confined to the cities only, but has spread through the villages and rural districts.”
Educated apologists like Justin Martyr, Origen, and Irenaeus claimed the name of Christ, as did those whom the Roman critic Celsus sneeringly described as, “wool-workers, cobblers, laundry-workers, and the most illiterate and bucolic yokels.” What the educated pagans considered a weakness, the early Christians regarded as a strength: the early church gladly included members from all walks of life.
Ben Franks is an MDiv student at Puritan Reformed Theological Seminary. A native son of the PCA, he has done mission work in England with the EPCEW and served with churches in the PCA and OPC. He studied at Patrick Henry College and completed his B.A. in Classical Christian Education through Whitefield College. His writings have been published in the Puritan Reformed Journal, the Confessional Presbyterian Journal, and the Banner of Truth Magazine.
"What Would Justin Martyr Do?" by Richard Winston
"A Church Growth Discipline" by David Prince
"Teaching Church History to Children" by Simonetta Carr
Only One Way, ed. by Richard Phillips and Michael Johnson
Histories and Fallacies by Carl Trueman
 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.
 For a fuller discussion of the range of scholarly opinion on the subject, see Stark, “The Rise of Christianity,” 3-27. Also useful is the overlapping discussion in Rodney Stark, Cities of God: The Real Story of How Christianity Became an Urban Movement and Conquered Rome, (New York: HarperCollins Publishing, 2006), 63-70. It should be noted that Stark is probably too quick to reject accounts of mass conversions as a factor in Christian growth. Nevertheless, his basic numbers and approach are useful and provide a framework for understanding the scholarly consensus on the growth of the Christian population in the Roman Empire.
 Schaff, ed., Ante-Nicene Fathers, 3:45. No doubt, Tertullian stretched the point for rhetorical effect when he added a few lines later that, “almost all the inhabitants of your various cities [are] followers of Christ.”
 See Wayne A. Meeks, The First Urban Christians: The Social World of the Apostle Paul, (New Haven, CT: Yale University Press, 2003).
 The reason for these dynamics is not hard to understand. Since sea travel was the most common mode of transportation, port cities were among the first to receive word of the Christian message and since cities had more people there was a larger pool of people to constitute a church. See Stark, “Cities of God,” 70-83.
 Stark, “Cities of God,” 225-226.
 See the groundbreaking recent work of Thomas A. Robinson, Who Were the First Christians?: Dismantling the Urban Thesis, (Oxford, UK: Oxford University Press, 2017).
 As quoted in Michael J. Kruger, Christianity at the Crossroads: How the Second Century Shaped the Future of the Church, (Downers Grover, IL: Intervarsity Academic Press, 2018), 11.
 As quoted in Robert Louis Wilkin, The Christians as the Romans Saw Them, (New Haven, CT: Yale University Press, 2003), 97.
 For further discussion of the sociological and demographic make-up of early Christianity, see Kruger, “Christianity at the Crossroads,” 11-39, and Stark, “The Rise of Christianity,” 29-48.