I'm familiar with Biblical history, the evangelism of Paul, and the legitimization of Christianity by Constantine. But I don't under how the religion spread so far and fast during the first few hundred years before Constantine ended the persecutions. There were so many other religions and so much violence against Christians. Not to mention the fact that gentiles weren't even sure the religion applied to them for most of the first century.
I've written a couple of related posts such as here, but the main point to make is that you don't need a very high or unusual growth rate for Christianity to make it to 10% by the time of Constantine. Rodney Stark devotes a chapter to it in The Triumph of Christianity, in which he takes a population of 1000 Christians, correlates with other demographic studies, and suggests a model based on 3.4% growth, which he notes is not that high, "many contemporary religious bodies, including the Jehovah's Witnesses and the Mormons, have sustained well-documented growth rates as high as or higher than 3.4 percent a year for many decades".
Such a projection get you to around 6 million in the year 300, which is around 10% of the population.