Ramsay MacMullen, Changes in the Roman Empire: Essays in the Ordinary
(Princeton: Princeton University Press, 1990), p. 136 (end-notes omitted):
What Christianity put forward was the fearful novelty of a God who would burn them alive in perpetuity for their very manner of life, spying out their transgressions wherever committed, as he would correspondingly reward the virtuous. Beginning with John the Baptist's and Jesus' preaching, on through Paul's acknowledgment of "the wrath to come," the flames of hell illuminated the lessons of Christianity quite as much as the light of Grace. Actual scenes of speeches delivered to non-believing crowds show that the message was made plain, for example, by Paul at Iconium very much as Jesus had told His disciples to do; and we know that it got through, at least to Celsus. He remarks that Christians "believe in eternal punishments" and "threaten others with these punishments." Clearest of all is the scene in the amphitheater at Carthage where the martyrs, referring to their coming torment, tell the crowd by sign language, "You, us; but God, you"; but Pionius had elaborated on similar comparisons and warnings of condemnation and suffering, in the city square of Smyrna. It is likely that this particular article of faith was as widely known as any outside the Church. Despite the Apologists' attempts, however, to make eternal hell-fire credible by reference to Tartarus or to Stoic predictions of universal conflagration, non-believers found it novel and hard to accept.