Stark, Rodney. "Chapter 6: Christianizing the Urban Empire: A Quantitative Approach." The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries. San Francisco: Harper Collins, 1997, 129-145..
Christianity rather early became a largely urban movement. Stark considers the sociological setting of urban areas around 100 to see why this might have happened (Stark 1997, 129). Stark observes that actual demographic information about many cities is elusive. However, he cites a work by Chandler and Fox (Three Thousand Years of Urban Growth, 1974) which gives plausible data for 20 Greco-Roman cities in 100 (Stark 1997, 131). Stark adds Athens and Salamis to the list, giving his own estimates of population.
The spread of Christianity may be reasonably estimated based on when a city is known to have had a church. Stark assigns the number two to those with a church by 100. He assigns a one to those with a church by 200. He assigns a zero to those with no church by 200 (Stark 1997, 134). The larger the city, the more likely we would think a new religion like Christianity would be found. This is at least partially true, though Stark does not find an overwhelming correspondence.
Travel in imperial Rome was relatively easy, so Stark does not think distance from Jerusalem is likely as a deciding factor in how soon Christianity reached a community (Stark 1997, 135). Stark calculated distance using known trade routes, rather than any sort of straight line measurement. He also considered how closely monitored by Rome a community would be, based on its distance from Rome (Stark 1997, 136).
Stark further considers the level of cultural continuity in the spread of Christianity. People in areas with Jewish population would be more likely to recognize Christianity as something not entirely foreign (Stark 1997, 137).
Stark finds that having a Jewish presence in a city made the growth of Christianity more likely. Proximity to Jerusalem was also a major factor, but possibly for cultural, rather than geographical reasons (Stark 1997, 138). The presence of a synagogue was very important. The more Romanized a city was (with the exception of Rome), the later it was to become Christianized (Stark 1997, 140). Stark also notes a strong correlation of Christianity and Gnosticism. He does question whether Gnosticism arose as a Christian heresy or a Jewish heresy. Because Christianity alone has a demonstrable impact on the spread of Gnosticism, Stark considers it to be a Christian heresy (Stark 1997, 142).