Christianity in the roman empire.

[Thiudareiks Flavius]

In regards to when exactly the Roman Empire became majority Christian, I decided to check and see what Gibbon had to say. Gibbon estimates that only 20% of the population was Christian by the time of Constantine. Like others have mentioned, he also points out that some regions and cities had a much higher proportion of Christians than others (Near East vs. Gaul, for instance).

It looks like he thinks that the Empire was majority Christian by about a generation after Theodosius – say about 420 or so.

Well, Gibbon isn't exactly cutting edge research and we have materials and approaches available to us now that he couldn't have dreamed of. Estimates of the per centage of Christians in the Empire when Constantine came to the throne vary, and range from 5 to 15, but rarely as high as 20%. And recent studies indicate that the majority had converted by the late Fourth Century, not the early Fifth. The best book on the subject is Rodney Stark's The Rise of Christianity. Stark uses a combination of historical analysis, social research approaches and the study of more recent, well-documented conversion rates to estimate how Christianity grew. Working from a reasonable estimate, based on sociological research, of a 40% growth rate per decade, Stark traces the growth of Christianity from 40 believers in 30 AD to 6 million in 300 AD, with most of this growth coming from social networks of family and friends converting family and friends.