As I mentioned in my response to Christianity Today, one of the central failures of our discussion about Christendom was our conflation of the political and cultural aspects of Christendom. This is, in many ways, understandable. The political and the culture have a complex relationship, and drawing a clear line between them makes me suspicious that their relationship has been oversimplified.
Those who criticize Christendom, regardless of its formulation, often do so on grounds that the Church shouldn't be in the business of seizing power. And on this point, they are right. But if Oliver O'Donovan's explication is accurate, Christendom begins not when Christians seize control, but when the political authorities acknowledge the reality of Christ's Lordship. In Desire of the Nations, he states the relationship this way:
The rulers of this world have bowed before Christ’s throne. The core-idea of Christendom is therefore intimately bound up with the church’s mission. But the relationship between mission and Christian political order should not be misconstrued. It is not, as is often suggested, that Christian political order is a project of the church’s mission, either as an end in itself or as a means to the further missionary end. The church’s one project is to witness to the Kingdom of God. Christendom is response to mission, and as such a sign that God has blessed it. It is constituted not by the church’s seizing alien power, but by alien power’s becoming attentive to the church.
Yet (seemingly contrary to the above), O’Donovan goes on to say that the Church and State are to exist in a relationship of mutual service, where the state helps the church's mission either by respecting its own limitations and maintaining, interestingly, a self-awareness that it is temporal and transient, or by a “willingness to listen to [the church’s leaders] as they explain the church’s task.” In taking this latter route, the state neither coerces nor steps over its temporal boundary. As O’Donovan points out elsewhere, this is the logic of 1 Timothy 2:1-4:
First of all, then, I urge that entreaties and prayers, petitions and thanksgivings, be made on behalf of all men, for kings and all who are in authority, so that we may lead a quiet and tranquil and quiet life in all godliness and dignity. This is good and acceptable in the sight of God our Savior, who desires all men to be saved and to come to the knowledge of the truth.
O'Donovan is rightly concerned that a confused implementation of Christendom can lead to the faulty view that the mission of the Church has come to an end. But for Paul, the logic works the other direction: the states humble acknowledgment of its own limits gives space for the Church to live out its mission.
It is, I think, this articulation of Christendom to which I would tentatively ascribe. Christendom is a Churchly reality first, but a Churchly reality that bears political fruit.