In one of those serendipitous encounters that make life more enjoyable, I recently found myself reading "The Rise of Christianity" by Rodney Stark
As an Italian, I studied my good deal of Roman and Christian history at school but I had never grasped how incredible, and totally non-obvious, an achievement was for this "obscure Jesus cult" to take over one of the vastest empires in history in merely 300 years. The question sets the scene for an exciting read, but on top of that, the author does an incredible job in keeping it condensed within 200 pages of clear sociological explanations without tedious enumeration of facts. I recommend the book to everybody interested not only in this specific chapter of human history but in the broader topic of how disruptive social and cultural change happens.
At the core of the book lies a simple yet ofter overlooked fact: any movement or phenomenon that can keep growing at a steady rate for a sufficiently long period of time will become dominant. Compound growth, as noted by Einstein (apocryphally) is the most powerful force in the universe. That's the obvious part. The non-obvious one is how to maintain said growth rate.
In Christianity's case, it was essentially a matter of "fit". Multiple aspects of the emerging faith made it the perfect killer of the incumbent Greek-Roman tradition. From the focus on family and procreation (amidst a largely unmarried and low natality population) to the commitment to charitable endeavours (which made the Christian population more resistant to epidemics), to their openness and ability of assimilation.
History takes often unique turns and we have no guarantee that the outcome would be the same if we could replay it from the start. The history of Christianity shows, however, that certain innovations are simply too powerful to be stopped. It seems like nothing is changing, until it changes forever.