I just read Rodney Stark's The Rise of Christianity. Stark tries to shed light on the rise of Christianity in the Roman Empire by combining the rather fragmentary evidence from ancient times with contemporary sociological research on the way religious communities develop and spread. In some ways, the book is sufficiently pro-Christianity that it should satisfy Christian readers. For example, the last chapter of the book argues that "the [Christian] religion's particular doctrines... permitted Christianity to be among the most successful and sweeping revitalization movements in history." Then, after discussing the grotesque spectacle of the Roman games-- where gladiators killed each other and prisoners were fed to wild beasts; much of the book discusses the cruelties of ancient Roman society-- Stark concludes: "Finally, what Christianity gave to its converts was nothing less than their humanity. In this sense virtue was its own reward." (p. 215)
What would make Christians most uneasy, I think, is Stark's insistence that religious conversion spreads exclusively through social networks, a lesson that he draws from observing the experiences of new, rapidly-spreading religions such as the Mormons and the Moonies. It turns out that, based on the best statistical reconstructions, the rate of growth of Christian membership in its first three to four centuries was about the same--40 percent per decade-- as that of the Mormons has been. I think a lot of Christians, including myself, would be uneasy seeing early Christianity-- the truth, in our view-- classified alongside lots of false contemporary sects, and treated as subject to rules which can be observed and then applied elsewhere like physical laws. One would hope that truth would play a role in the conversion process, and that conversion to the One True Faith is a wholly different kind of process than falling away into some heresy.
Maybe that's just partisanship. Still, in the absence of a definition of "religion"-- Stark offers none-- some skepticism about making and extrapolating from generalizations about "religions" seems warranted. In his book The Everlasting Man, which covers some of the same historical territory as The Rise of Christianity, G.K. Chesterton argues that the Christian Church is an entirely unique entity, and that "comparative religion" is an illusion. Chesterton's book is an apologia for the faith, not a work of social science, so social scientists may regard it as outside their discipline. Still, I would like to see some social scientist try to cope with Chesterton's argument, by, for example, offering a definition of religion which would include all those things, and none others, that we usually call religions. I suspect it could not be done; and the failure to do so could even be a useful input into the American legal process, and particularly, in helping to phase out the discrimination for-and-against religion which has become a feature of American curriculum design. (I say "for-and-against" because, while "religious" teachings, of even the most generic kind, are excluded, there is also a sort of kid-gloves policy where philosophers and scientists who regard atheism as axiomatic have to keep that opinion out of the classroom, more or less.)
One thread in Stark's story seems dramatically presentist: opposition to abortion and infanticide appears to have been a principal driver of the growth of Christianity, because it led to higher birthrates and a lack of gender imbalance. But Stark is not pandering to the religious right: it is, it seems, a historic fact that abortion and infanticide-- in particular, exposure of girls-- were widely practiced in the ancient Roman Empire, but completely forbidden by Christians.
Reading Stark, I am constantly struck by the parallels between the rise of Christianity in ancient Rome and the rise of Christianity in contemporary China. For example, one parallel is the new religion's overrecruitment of women in a society which had a surplus of men due to sex-selection infanticide (in Rome's case) or abortion (in China's case). Another parallel is in the way that lackadaisical persecution helped growth in the church by creating opportunities for heroic demonstrations of faith, without being severe enough to totally wipe out the church.
My own conversion experience might not count in Stark's terms, because I converted, not to a totally new faith, but from one form of Christianity-- Mormonism-- to another-- Orthodoxy-- even if Mormonism differs from Christianity in enough ways that most (traditional) Christians would be ambivalent at best about recognizing Mormons as Christians. In one way, my story contradicts Stark's insistence that religions spread almost solely through social networks: I knew about Orthodoxy and was drawn to it long before I met any actual Orthodox. On the other hand, I didn't join until I found a church with welcoming people to meet, and that meeting was somewhat happenstance, not really a result of deliberately seeking them out. But then, I went to lots of churches with welcoming people in them, which I didn't join on doctrinal grounds. In my case, social networks and convincing doctrines were both necessary conditions.