This is another, rather more lengthy, response to Wedgeworth. I am rather more sympathetic to Brad Littlejohn's position, which is here.
The chief difficulty I have with Wedgeworth and his friends' model is that it seems to divide the human person into two persons. I would be willing to allow that the civil magistrate is a deacon within the Church, responsible for keeping order in the Church, but the civil magistrate is then not a magistrate within a separate kingdom from the Church, but a petty official within the City of God. (At least the believing magistrate is.)
But when we say things like: "Furthermore, man is always both body and spirit, and he always has bodily concerns and spiritual concerns. Thus man needs to order both his body and his spirit...Thus it should not be thought unusual for us to say that there are both bodily and spiritual sorts of politics. Man is always concerned with both, and thus he fashions representative governments for both. The magistracy rules the bodily realm, and the ministry rules the spiritual realm." we seem to divide the human person into two persons, and moreover, to divide even the city of God into two.
The clearest way I can think of to get at this problem is to adopt the terminology from the Christological controversies, with the question being is Christ from two natures, or merely in two natures. The Miaphysites staunchly maintained Christ is not merely in two natures, but is indeed one from two natures. They accused the Calcedonians of separating Christ into two, and thus embracing de facto Nestorianism--as if Christ existed over here, and separately, over there. Ultimately, the orthodox solution was to embrace both positions, thus St. Maximos the Confessor can say Christ exists in and from two natures.
But as I read it, Wedgeworth's and Escalante's doctrine makes precisely this error the miaphysites accused the orthodox of making. Namely they divide the human person into two persons, one existing here, the other there. They say, in short, that the human person exists in two natures, but implicitly deny there is one human nature from these two natures.
This is a crucial mistake, first, because it denies the unity of Christ. Even in the Eschaton, human nature shall be body and spirit. And if human nature is merely in two, and not from two, even in the Eschaton it shall be in two, not from two. Thus the two kingdoms are not a temporary feature of the New Covenant which is destined to perish with use. Rather, even after the Resurrection, there shall be physical and spiritual politics. But it is only possible to have two polities with two rulers. And therefore the One Christ shall not be one ruler, but shall be two rulers. But Christ is not two Christs, but One. And thus, the hypothesis that we need spiritual and physical politics results in a contradiction, and therefore is false.
N.B. I am not simply taking a Lutheran criticism of the Reformed doctrine of the Incarnation. I do not mean that Wedgeworth's doctrine separates the humanity from the divinity, but rather that it separates even the humanity into two.
Second, it is novel. During the Christological controversies, the Miaphysites were able to say that the orthodox were Nestorian because they claimed that unlike human nature, which is one, though it is from two, Christ's nature is not one. That is, both sides agreed that human nature was not parted into twain, but rather was one composed out of two. This was simply not an issue. Both sides agreed that human nature was one--otherwise how could the Miaphysites have appealed to the unity of human nature as an illustration of their position?
Finally, it is unAugustinian. For Augustine, the City of God consisted of those who were turned by love--on earth as in heaven--rather than by lust for power. But the citizen of the City of God was turned by love in all his affairs, bodily and spiritual. The citizen of the City of Man was turned by lust in all his affairs, bodily and spiritual. Augustine's division is not between the body and the soul, but between charity and lust. Though Wedgeworth's distinction maintains some of the Augustinian concerns, it is radically, a different doctrine.
Rather than saying that human nature exists in two, we rather should say that human nature is from (or in and from two). And if it is from two, we exist in one kingdom—or rather inasmuch as we exist in two kingdoms, it is because they are hostile to each other. Thus St. Paul insists that the Philippians—Roman citizens—do not have human citizenship, but citizenship from heaven. And the Romans, correctly, perceived this challenge to their city. Though the Christians were obedient to the Emperor, they would not recognize the legitimacy of the emperor by offering incense to him, but rather offered incense to the true emperor, Jesus Christ. This is not to say that should the Emperor convert, he must abdicate, but rather that he must recognize that his kingdom is a temporary province of the true Empire, the Church.
Moreover, as N. T. Wright has shown, we are colonists from heaven. Now Roman colonists were farmers sent to till the land, and by planting and harvesting the seeds, turn the land into Roman land. And this is precisely the Christian mission. We are commissioned to labor for the kingdom of heaven, and through our labors and prayers to bring the kingdom onto earth. And we labor for heaven, turning this world into heaven, through farming the land. “The blood of the martyrs is the seed of the Church.” We labor by planting our bodies as seeds in the land here, awaiting that day when the “at Christ’s glorious advent [this] holy harvest field shall all its full abundance yield , then the golden ears of harvest shall before His presence wave, rising in the sunshine glorious from the furrows of the grave.” (As the Christopher Wordsworth hymn says.)
And thus we come to the correct understanding of the fallen, but redeemed, human person. The fallen but redeemed man is in the world as Christ was in the world. That is, we are subject to corruption, but “belong body and soul”, to the kingdom of heaven. And we are, body and soul, a seed. And in Christ, we have become, body and soul, a living, heavenly seed. And as Christ’s body and soul was a living seed, and has risen again as the firstfruits, and has waved before the Father as a firstfruit offering, so we are, body and soul seed which shall grow into a heavenly person, humanly identical to Christ. We do not exist in two realms, but rather are one from two, and awaiting planting that we may sprout up, body and soul, into new life.
The ministers of the Church are given to guide this seminal Church, this seminal heaven, till that time when we, and she, shall spring out from the furrows of the grave, and wave before Christ’s presence. Likewise, when the civil magistrate submits himself to the Church—not to the magisterium, but to the Church—he takes a position within the Church, physically ordering the kingdom of heaven. Helping, in his way, to plant the seeds; as the pastors help in theirs.
Moreover, as an officer in the Church—unless the Church is fundamentally Macabean, not Davidic, which is a coherent position, in which case he would have a position in the Church, but merely a functional one, and not an official one at all—he should be subject to church discipline, particularly to the sort of discipline exercised against officers, namely, defrocking. And he specifically is not the ruler of the bodily realm—as if such a thing existed.