Why did Christianity go from a marginal sect to a dominant force in the Roman Empire?

For many years Rodney Stark was a Professor of Sociology at the University of Washington. He wrote an introductory college sociology text that has been through ten editions. 

A fascinating book he published in 1996 is The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries 

In the preface, Stark states his approach to trying to answer the question that is raised by the book title.

What I am primarily trying to contribute to studies of the early church is better social science—better theories and more formal methods of analysis, including quantification wherever possible and appropriate. Thus in this book I shall try to introduce historians and biblical scholars to real social science, including formal rational choice theory, theories of the firm, the role of social networks and interpersonal attachments in conversion, dynamic population models, social epidemiology, and models of religious economies.

Stark then considers how the role of women, caring for the sick in epidemics, social networks, martyrs, urban chaos, crises, and other factors all lead to the exponential growth of the church over three centuries.

                          Domenico di Bartolo, Care of The Sick

In the final chapter Stark states 

as I conclude this study, I find it necessary to confront what appears to me to be the ultimate factor in the rise of Christianity. 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.