Any good, standard history of Christian political ethics (like O’Donovan and O’Donovan, From Irenaeus to Grotius: A Sourcebook in Christian Political Thought, 150-1628) demonstrates that the two-kingdoms motif can be found in the church fathers, especially Augustine (see Robert A. Markus’ work), weaves its way in modified versions through the Middle Ages, and is given vigorous voice not only by Luther but also by Calvin and other magisterial reformers.
Augustine himself was more of a “one-kingdom” person early on, sharing his fellow-Christians’ confidence in the wake of Constantine’s cessation of persecution and adoption of Christianity as the religion of the empire. However, perhaps nudged by experiences (such as the sack of Rome by the pagans and the reproach of latent Roman pagans that abandoning the defeat was due to having abandoned the gods), Augustine rethought the relationship between the two kingdoms. He traced the “two seeds” after the fall, one from Cain (builders of civilization) and the other from Seth (the covenant line), to the reunion of cult and culture in the old covenant theocracy, and its division again in the exile.
The ethics of Jesus’ Sermon on the Mount are markedly different from the old covenant that God delivered to Moses on Mount Sinai. Now is not the time of driving out the enemies of God, but of praying for them and preaching the gospel. It’s the era of forgiveness and grace, not of judgment.
“Christendom” (the fusion of the two kingdoms) is the illusion that a common empire can claim the conditional promises that God gave uniquely to the theocratic nation of Israel. Repeatedly, Calvin asserted that these laws were given exclusively to Israel and are no longer binding on Christians or on nation states. “Christendom” is a tough habit to break when your church happens to be favored by the state and your sovereigns are anointed in religious ceremonies like David and his heirs.
So it’s not surprising that Christians prefer “one kingdom” when the wind of history is at their back (or so it seems, at least) and “two kingdoms” makes a comeback when the church is persecuted or no longer privileged. Some Christians today seem hostile to the two-kingdoms idea because it undermines some of the motivations for culture warring. America is a Christian nation and it’s losing its Judeo-Christian identity. We need to renew our national covenant with God. This is the assumption that I hear from some brothers and sisters in their visceral reactions to this concept. Ironically, many liberal Protestants react for similar reasons. If Christ’s kingdom is not to be identified with the church’s work of transforming societies, cultures, economies, and political orders, then what else is it for?
I’m not saying that the only reason that the two-kingdoms doctrine is unpopular among Christians is a vague but symbolically powerful cultural dominance. However, the history does suggest some kind of connection to the political winds of the times.
With all due respect to Lutherans, it was Calvinists who argued most strongly for the independence of the church from the state in Geneva, London, Amsterdam, and elsewhere, and defended religious liberties. Among evangelical Protestants at least, Calvinists were directly involved in arguing (along with Quakers and deists) for the separation of church and state. Trained under Presbyterian stalwart John Witherspoon (a signer of the Declaration of Independence), James Madison used two kingdoms arguments for his case. In fact, he surveyed history to argue that the church itself is healthiest when it is least dependent on state sponsorship and support.
Clearly, Luther drew the lines between the two kingdoms in clear, bold colors, but so did Calvin—and both did so especially over against the radical Anabaptists who were trying to take over cities in the name of Christ’s millennial kingdom! Calvin wrote explicitly of the “two kingdoms”: both under the reign of the risen and ascended Christ, but “in different ways”; one, by common grace and the moral law inscribed on the conscience and the other by saving grace and the gospel. Neither Lutherans nor Calvinists have been consistent in working out their theory, but the two-kingdoms doctrine has a substantial body of reflection throughout the whole history of the church.