The thesis of The Mighty and the Almighty: “Central to a Christian theological account of the state is an understanding of the duality of state authority mediating divine authority and an aunderstanding of the duality of Christians being under the authority of both church and state” (35).
A key question regarding his second duality is the manner in which that dual membership coincides in the individual who is both citizen and Christian. Church membership is compatible with membership in secular political communities, but it also trumps that membership. There are many imaginable scenarios in which a prima facie duty laid upon John as a citizen would be trumped by an ultima facie duty laid upon him as a Christian, but there is no Christian duty that can be overridden by appeal to the duties of a citizen.
Augustine was the church’s first great theorist of this difference; the city of God endures everlastingly, and as the totus Christus, belongs to the eternal intentions of God’s creative love. The city of man, by contrast, is an accident of human history, a remedy for sin, ultimately bound for dust and ashes. The city of God belongs to the history God wills, as the community of those gathered around his Son; the city of man belongs to the history that God permits, as the loving restraint that saves sin-sodden humanity from bringing itself utterly to nothing.
Speaking anachronistically, we might say this: for Augustine, the city of God is a natural kind, whereas the city of man is an artificial kind. The former is God’s idea, the latter man’s (or perhaps most immediately, Satan’s). Chuck Mathewes captures this distinction nicely in The Republic of Grace: “Christianity’s perspective is neither finally nationalist nor cosmopolitan…Proper Christian formation challenges the way we take the division of the world into nation-states as ‘natural’” (30). From the viewpoint of the church, states come and go, each a manifestation of God’s providential restraint upon our sin, but each no more than a tool in his hand, each and all destined eventually to wear out, to be consumed as dross in the purging fires of the end.