I have been laying out the Prospectus for my proposed book The Triumph of Christianity that I circulated to several publishers last summer, and I am now at the very heart of the matter, the explanation (as I saw it then) for why Christianity succeeded so massively in the Empire during its first three centuries. Here is what I said:

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There were two key factors specific to Christianity that facilitated the growth of the Christian church. These two factors could not be found in other religions of the empire. And in tandem – this is a very important point: they worked in tandem – they led to the spread of Christianity and the demise of all the other religions of the empire. The factors: Christianity was evangelistic and it was exclusivistic.

Religions in antiquity were not evangelistic, in part precisely because they were not exclusivistic. A broad survey of ancient writings makes it completely clear: most people did not much care whether you adopted their religious practices or not. No one insisted that you begin to follow their religion – and most especially only their religion. All the gods deserved to be worshiped, yes. But if you worship your own gods, there is no reason that you should have to worship mine. Jews were a partial exception, but only a partial exception. Most Jews refused to worship anyone else’s gods. But they did not insist that others convert to worship their God. Christians, on the other hand, did insist on this. Quite vociferously. Christians from the outset were inherently evangelistic.

This in no small measure was because of the very nature of Christianity. Christians, unlike most other religions…

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Christians, unlike most other religions (again, with the partial exception of Judaism), were concerned about life after this life. They were concerned about eternal life. They believed that there would be life after death. And they believed that Jesus’ death and resurrection were the way to secure life after death. In fact, he was the only way. Anyone who did not accept the way of Jesus would be lost. Forever. And so Christians felt the urge to convert others to worship Jesus, as the Son of the one Creator god.

This emphasis on the afterlife had an obvious attraction for potential converts. As with all other religions, there was certainly a clear and compelling concern in the Christian faith for securing divine power to help one survive and live well in the present life. But the divine power available to the followers of Christ continued in the life after death as well. Those who experienced hardship now, even hardship in extremis (e.g., persecution, torture, and death), would be vindicated in the life to come and rewarded with eternal bliss. At the same time, the fate of those who refused to accept this Christian message – as this message was “proved” by the miraculous events that had occurred in the life of Jesus, in the lives of his apostles, and in the lives of his followers in the present – would be horrible and unthinkable: eternal torment. Everlasting joy or everlasting torment: Christians repeatedly emphasized that everyone could make the choice, and the choice involved a complete conversion away from the “dead” gods of the pagan religions to worship the living and true God as revealed by Jesus.

And so it is for more or less that same reason that — unlike any other religion in the empire — Christianity was exclusivistic. Christians from the very beginning insisted that there was one way to have this blessed afterlife and to avoid the horrors of hell. It was not simply by being a good person, as so many other people at the time imagined. It was by faith in Jesus. And only by faith in Jesus. For that reason, anyone who began to worship Jesus had to abandon their previous religious ideas and practices . Conversion was to be complete, total, and exclusive. Christians believed that if they were right (and they were highly convinced that they were indeed right), everyone else was wrong. This was Jewish monotheism raised to a new level.

It is largely the combination of these two factors that led to the ultimate triumph of Christianity. Christianity did not need massive conversions at any one time to succeed. It only needed steady growth. And that is because it destroyed all the other religions as it spread. It was the only religion to do that.

This is a central point, and it can be easily explained. As I’ve already indicated, anyone in the Roman world who was a worshiper of Zeus who decided, later, to become also a worshiper of Apollo, did not stop worshiping Zeus. He simply worshiped both of them. Along with other gods of the state. And of his city. And of his family. And so on. But anyone who worshiped any of these gods who started to worship Jesus had to give up the other gods. And so when someone who was already a pagan began worshiping other gods, they remained pagan. But if they started worshiping the Christian God, they had to leave paganism.

A social historian of religion from Yale, Ramsay Macmullen, author of Pagans and Christians, explains how it worked. Suppose there were two preachers trying to convert a crowd of 100 pagans to adopt a new form of worship. One of the preachers was proclaiming Athena, and the other Jesus. And suppose both preachers were equally successful, convincing half the crowd. If that happened, then paganism would lose 50 adherents and gain no one; Christianity, on the other hand, would gain 50 adherents and lose no one. The Christian mission destroyed all other religions in its wake. (And to do so, Christians did not even have to be equally convincing as adherents of other religions; since the religion required only slow, steady grown to destroy the others. Christian missionaries could in fact be unconvincing to most people they talked with, and the church would still grow and paganism shrink.)

Christianity was the only religion that was annihilating others. Pagan religions were never exclusivistic and by and large they were not evangelistic. Judaism was not actively evangelistic either: most Jews did not much care whether anyone converted to worship their God. God was their God; the other religions could have their gods as well. But Christians did not see it that way. Faith in Christ is the one and only way to have salvation. Anyone who did not believe in Christ would be eternally punished. Christianity was right. All other religions – including non-Christian Judaism – were wrong. And so others had to be converted (evangelism). And when they did so they had to swear allegiance to the God of Jesus alone (exclusivity). Christianity was the only game in town playing by these rules, and it was these rules that would, over the steady course of years, decades, and then centuries, leave the Christian religion as the one dominant form of devotion in the empire.