Over the course of 300 years, the religion had grown to be about five percent of the population of the empire, or some three million adherents. That rate of growth does not require massive conversions but simply a steady stream of converts. It represents a growth of about 40 percent every 10 years (which happens to be the growth rate of the Mormon Church over the course of the 20th century; see Rodney Stark).
This growth was not achieved by massive evangelistic campaigns but by social networking, as one person who converted would then convert his spouse and (some of his) children, neighbors, and friends; over time, each of the converts would do the same.
– From Jesus to Constantine: A History of Early Christianity
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