Here is a taste: Estimating Christian numbers at any point in history (even today) is a difficult task. For the early church, the classic figures come from my Baylor colleague Rodney Stark in his 1996 book The Rise of Christianity. This made an excellent attempt at providing some kind of general parameters. Drawing analogies with modern new religious movements, Stark showed that Christians could have achieved their remarkable growth in the first few centuries by quite familiar forms of growth and conversion, without any claims to miracle or uniqueness. He estimated a global Christian population of 40,000 in AD 150, rising to 218,000 in 200, and 1.17 million by 250. According to his calculation, it was around 180 that global Christian numbers first surpassed the symbolically weighty figure of 100,000.
Stark would be the first to admit that those figures are anything but precise, but they provide plausible limits. If someone suggested a Christian population in 200 as ten thousand, or as ten million, then they would assuredly be wrong. But a range anywhere from (say) 150,000 to 350,000 would be quite plausible.
There are some reasons to place the figure for AD 200 a bit higher than Stark proposed. One specific issue concerns the total population with which Stark is working, which is that of the Roman Empire. His estimate for the overall Roman population is rather lower than more recent estimates, and Christian numbers must be adjusted accordingly. Also, it is never quite clear whether early estimates for Christians were taking account of the full spectrum of people who would have used that description for themselves, including all “heresies.
Read the entire piece here.