<p>This paper applies the economic approach to the history of Christianity in the religious environment of the Roman Empire, a ground already covered by Stark, Ekelund and Tollison. We show that Greco-Roman religion was not in decline and was not threatened by Christianity either in terms of numbers or influence, but it was a fragile system because it lacked a professional priestly class with an interest in its continuance; hence, when the emperors raised Christianity to state religion, it was doomed. Christianity rose slowly and inconspicuously, struggling to extricate itself from its Jewish apocalyptic beginnings; it was not a superior doctrine, and specifically not a superior afterlife promise, that drove its growth in its sectarian centuries. Rather, Christians were able to achieve a distinctive doctrinal identity and organizational unity only when Toleration offered them both opportunity and incentive to become a universal religion, and the emperors bet on that. On this reading, for all it was hell-bent on destroying paganism, Christianity emerged as its killer only by accident.</p>