My claim is that Augustine’s Two Cities and the Reformed Two Kingdoms ideas are compatible, not that they are identical. They are harmonious, but get at different aspects of the truth: Augustine’s Two Cities describe two eschatological peoples, one marked by love of the Creator above all and one marked by love of the creation above all; in this world the Two Cities mingle, but they can’t be identified with any particular earthly society or institution; there is stark antithesis between these Two Cities, and each person is a member of one city and one city only. The Reformed Two Kingdoms, on the other hand, pertain to the twofold way in which God rules this present world, primarily (for early Reformed theologians) through church and state. This means that Christians are actually citizens of both kingdoms. Christians, in other words, are citizens of two kingdoms, but of one city. As citizens of the city of God they stand in eschatological conflict with unbelievers; as participants in the common kingdom, they are called to co-exist in peace with unbelievers as far as possible...
My basic case in chapters 2–5 of LGTK is this: God gave the original cultural mandate to Adam as representative of the human race in an unfallen world, demanding perfect obedience and promising the attainment of an eschatological new creation as a reward for obedience. Adam failed and plunged the human race into a state of curse rather than eschatological blessing. But God sent his Son as the Last Adam, to fulfill God’s task for humanity perfectly and thereby to attain the new creation for himself and his people. Popular recent neo-Calvinist works speak of redeemed Christians being called to take up again Adam’s original cultural task (not to go back to Eden, but to fulfill Adam’s responsibility to fill the earth, have dominion, etc.). In response, I have argued that this cannot be the correct biblical paradigm for the Christian’s present responsibilities in this world. If Christ is the Last Adam, then none of us are called to be new Adams. It is not as if Christians have no cultural mandate (as Kingdoms Apart suggests I claim), but that the cultural mandate comes to the human race only as refracted through the covenant with Noah after the flood. It comes thereby to the human race as a whole (not to Christians uniquely) and is geared for life in a fallen world and holds out no eschatological hope of reward. Thus in order to understand our calling to participate in the life of politics or commerce, for example, we should understand these responsibilities as rooted in the Noahic covenant and as work to pursue in collaboration with unbelievers, as far as possible (without forgetting the different attitude, motivation, goals, etc. with which Christians take up these tasks). I also suggested that all of us who share a commitment to the Reformed doctrine of justification should appreciate the attractiveness of my suggested paradigm, built as it is upon traditional understandings of the covenant of works, the Two Adams, and the sufficiency of the obedience of Christ. This is an invitation to soteriologically orthodox neo-Calvinists to embrace a view of Christianity-and-culture that is more consistent with doctrines at the core of the gospel they love.
And from the Conclusion of his review:
It might also be interesting for a valiant defender of neo-Calvinism to address the following observation: most ordinary Reformed believers already live what might be called a two kingdoms way of life. When they follow the regulative principle of worship, uphold the church’s jurisdiction over its own discipline, and respect the Christian liberty of fellow believers in matters of faith and worship that are “beside” God’s Word (see Westminster Confession of Faith 20.2), they embrace aspects of Reformed practice historically inseparable from the two kingdoms doctrine. And when they live peaceably with their unbelieving neighbors—working, buying, selling, driving, flying, playing, and voting alongside them—are they not giving implicit witness to the reality of God’s distinctive common grace government over the world through the covenant with Noah? And if this is the case, then I suggest that the two kingdoms idea serves a clarifying function: it helps Reformed Christians understand in a more theologically clear way the Christian faith and life they are in so many respects already practicing.Two books by David VanDrunen on two kingdom theology:
Living In God's Two Kingdoms
Natural Law and The Two Kingdoms