How does Christendom related to Christ’s Kingdom?
For me, when we talk about “the Kingdom” we need to distinguish between 1) the Kingdom of Grace (the visible Church); 2) the Kingdom of power (all things under the authority of Christ for the good of the Church); 3) the Kingdom of Glory which is Christ’s reign in the New Heavens and New Earth.
In this age the Kingdom of Grace is holy colony of heaven on earth… an eschatological intrusion. The Kingdom of Power relates to the secular environment of the natural realm and the affairs of men outside of the church (including all providence). The Kingdom of Glory is our eschatological hope merging the holy and the common, the secular and the sacred into one absolute reign of Christ over a sinless creation.
When Darryl cautions us not to immantize the eschaton he is saying that we should not confound the holy and common prematurely (before the 2nd Coming of Christ). In this age the church is a kingdom apart from the kingdoms of this world.
Where does Christendom fit in? Well, it is not the Kingdom of Grace. Rather, it is the nations (part of the secular realm but within Christ’s providential reign over His Kingdom of Power) confessing that the moral order of the universe is in the hands of Jesus Christ.
This does not confuse sacred and secular. It does not combine the Kingdoms. It does not suggest wild eyes Puritanism, postmillenial transformationalism, or any other utopian vision of political possibility. It does not make the state a redemptive institution. It does not even make, necessarily, the state a just.
Rather, and quite humbly, Christendom simply reflects the facts of the universe as they truly exists.