Christianity did not grow because of miracle working in the marketplaces (although there may have been much of that going on), or because Constantine said it should, or even because the martyrs gave it such credibility. It grew because Christians constituted an intense community, able to generate the “invincible obstinacy” [against paganism] that so offended the younger Pliny but yielded immense religious rewards. And the primary means of its growth was through the united and motivated efforts of the growing numbers of Christian believers, who invited their friends, relatives, and neighbors to share the “good news.” . . .

Let me state my thesis: Central doctrines of Christianity prompted and sustained attractive, liberating, and effective social relations and organizations.

I believe that it was the religion’s particular doctrines that permitted Christianity to be among the most sweeping and successful revitalization movements in history. And it was the way these doctrines took on actual flesh, the way they directed organizational actions and individual behavior, that led to the rise of Christianity. – from The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries (1996)