To see the radicalism of Christ’s idea, I turn to the ancient Roman writer Celsus, who in the second century AD wrote an influential attack on Christianity. Celsus’s work was lost, but the church father Origen published a refutation, Contra Celsum, that helps us reconstruct his argument. Celsus basically accused the Christians of being atheists.

He was serious. For the ancient Greeks and Romans, the gods a man should worship were the gods of the state. Each community had its own deities—it was a polytheistic age—and patriotism demanded that a good Athenian make sacrifices to the Athenian gods and a good Roman pay homage to the gods of Rome. The Christians, Celsus fumed, refused to worship the Roman gods. They did not acknowledge the Roman emperor as a god, even though Caesar had been elevated by the Roman Senate to divine status. Instead the Christians insisted on worshipping an alien god, putting their allegiance to him above their allegiance to the state. What blasphemy! What treason!

I am not suggesting that the ancient Greeks and Romans were especially “religious.” Gibbon reports that philosophers and public officials held very different attitudes toward the gods than did ordinary citizens: “The various modes of worship which prevailed in the Roman world were all considered by the people as equally true, by the philosopher as equally false, and by the magistrate as equally useful.” Even so, religious identity in the ancient world was indissolubly tied up with your tribe and community. You could not be a good Dinka and not worship the Dinka deities, whether rock or stone or sun. Nor could you be a good Roman and not exalt the Roman deities, whether Apollo, Bacchus, or Jupiter.

Christianity introduced not only a new religion but a new conception of religion. So successful was this cultural revolution in the West that today the ancient paganism lives only in the names of planets and for those who follow astrology charts. Atheists do not bother to disbelieve in Baal or Zeus and invoke them only to make all religion sound silly. The atheists’ real target is the God of monotheism, usually the Christian God.