Jesus proclaimed the Kingdom of God but what he meant by the phrase it is not very clear to someone picking up the New Testament today. The Kingdom of God is probably best understood as “the space where God is king, i.e. where his law is obeyed.” Hence the words of the Lord’s Prayer: thy kingdom come / thy will be done. Jesus is in effect saying that his preaching (and in a way his very person) are the way to understand God’s will and do it. The Kingdom is not a political reality (not as we commonly understand it) but it is a social reality: it is the community of his followers.
Not to identify the Kingdom of God with the church, at least not the church understood as collection of office-holding clerics and religious hobbyists. But the church can be understood as the Kingdom of God insofar as the disciples of Christ are “the space” in which God is king.
The Kingdom of God is not presented as a static reality, but as a dynamic and growing one. It is something started by Jesus, and which grows to some sort of fulfillment.
If the Kingdom of God has a social element, then how does it relate to the political sphere?
When the New Testament was written the church was a mistrusted minority, and this has remained the normal status of the church in many places. The Kingdom therefore is one that can exist in conflict with the surrounding environment or in spite of the political order.
The church found itself in a different position in Europe from the time of Constantine but the main change was that it started to expect the state to promote the interests of the church. Laws that particularly offended Christian sensibilities might have been changed but it is interesting to note that Christians never expected the Christian state to enforce Christian morals. For most of the history of Christian Europe sins like prostitution, drunkenness, or sodomy were not considered subjects of state legislation.
The role of the “Christian monarch” of the Middle Ages was not terribly different from Paul’s vision of role of the pagan Emperor, punishing evildoers for the sake of keeping the peace, the main addition being punishing heretics, basically for the same reason. He would also personally promote the institutions of the church, generally out of his own pocket.
The modern state however is capable – or at least seems capable – of much more, whether for good or ill. Add to that the myth of progress, the narrative that humans can be made to behave in a more rational manner with the passage of time, and the efforts of the modern state to improve humanity seem to dovetail nicely with the notion of a dynamic and growing Kingdom of God.
The movement of the modern state towards its goals is therefore seen as part of God’s plan to expand his Kingdom, hence Progressive Christianity. The Progressive Christian allies himself with state power in order to achieve what he is told is progressive (never mind the fact that the goals of ‘progress’ change every ten years or so.) They concentrate on the fact that the stated goal of the modern state is the betterment of mankind through various services, and ignore that the unstated goal is the consolidation of power and control.
Of course every age has had Christians who wanted to identify God’s Providence with the machinations of politics, at least since Constantine. They are always disappointed in the end.