Along the same lines mentioned a while back…
There’s a Wikipedia article on the book:
The Rise of Christianity
The Rise of Christianity (subtitled either A Sociologist Reconsiders History or How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries, depending on the edition), is a book by the sociologist Rodney Stark, which examines the rise of Christianity, from a small movement in Galilee and Judea at the time of Jesus to the majority religion of the Roman Empire a few centuries later. Stark argues that contrary to popular belief, Christianity …