Given our starting number (1000 in 40 BCE), if Christianity grew at the rate of 40 percent per decade, there would have been 7,530 Christians in the year 100, followed by 217,795 Christians in the year 200 and by 6,299,832 Christians in the year 300.This is a plausible rate of growth, according to Stark. Mormons, for instance, have grown at a 43 percent clip since their beginnings. Therefore there is no need to seek miraculous explanations for the expansion, especially during the last half of the third century:
...But because of the rather extraordinary features of exponential curves, this probably was a period of "miraculous-seeming" growth in terms of absolute numbers...He has an interesting take on what this growth means for understanding the conversion of Constantine:
...Looking at the rise of a Christian majority as purely a function of a constant rate of growth calls into serious question the emphasis given by Eusebius and others to the conversion of Constantine as the factor that produced the Christian majority (Grant 1977). So long as nothing changed in the conditions that sustained the 40-percent-a-decade growth rate, Constantine's conversion would better be seen as a response to the massive exponential wave in progress, not as its cause.It is really helpful to see plausible numbers of growth and an explanation of how it might have happened. And I find it very interesting to place the conversion of Constantine in the context of this growth. I have always assumed that it was his conversion that accelerated the numerical growth of Christianity, but it is quite possible, as Stark suggests, that the laws of numbers were working themselves out and the exponential growth simply kept going after Constantine's conversion. But it would have seemed to those living through it that his conversion accelerated the process.
This interpretation is entirely in keeping with the thesis developed by Shirley Jackson Case in his 1925 presidential address to the American Society of Church History. Case began by noting that attempts by the emperor Diocletian in 303, and continued by his successor Galerius in 305, to use persecution to force Christians to support the state had failed because "by the year 300 Christianity had become too widely accepted in Roman society to make possible a successful persecution on the part of the government". As a result, Case continued, by 311 the emperor Galerius switched tactics and excused the Christians from praying to Roman gods, and asked only that they pray to "their own god for our security and that of the state." Thus Constantine's edict of toleration, issued two years later, was simply a continuation of state policy.
More to come on that I found very interesting on where the growth happened and how.