Blues Brother....Yes, it says in Acts 2:41 that there were over 3,000 Christians in Jerusalem in c. AD 33, and in 4:4 the number has increased to 5,000 sometime between AD 33-36, and then there were "tens of thousands" of Jews who converted to Christianity in Jerusalem in c. AD 58 (21:20). The problem is that the population of Jerusalem in the first century AD was only about 20,000-40,000 persons (swelling over a hundred thousand only during Passover), so the rate of increase indicated in Acts would suggest that Jerusalem would have become a Christian city by the time it was destroyed in AD 70, and this was not the case. So the figures are treated as demographic exaggerations like many other questionable figures in the OT, contemporary Greco-Roman writings, and even in Josephus (cf. the similar exaggeration that Christianity had already spread to "all creation to the ends of the earth" in Romans 1:5-8, 10:16-18, 1 Thessalonians 1:8, Colossians 1:5, 23). Moreover, there are more solid figures for the third century which shows that it was then that the Christian population increased from 200,000 to over 6 million in the Roman Empire, at a rate of about 40% per decade, and the same rate retrojected back in the second and first centuries would yield about 7,500 Christians in the year 100. Of course, one would not necessarily expect a uniform rate, so such a figure should probably be treated with a grain of salt, and it's all guesswork anyway, but I just wanted to clarify that a good number of scholars estimate a much lower population than the "conservative" estimate indicated here. Compare also Origen, Contra Celsum, 3.10: "It is obvious that in the beginning Christians were small in number". Population estimates also take into account other factors like city size, residency patterns, food production, and social structure of house churches. See more in the Hopkins paper, or the paper "The Population of Western Palestine in the Roman-Byzantine Period" by M. Broshi in BASOR, 1979.
