The Church and the Kingdom are not the same thing, and let me explain why this is important as we continue to address the question of violence.
As Peter Leithart shows in his book Against Christianity, the Church is presented to the world, not as an alternative club for religious people meeting on Sundays, but as a new polis, a new way of living out our true humanity in Jesus. Now if this were taken woodenly at face value, right across the board, then it means that the Church, the City of God, will eventually have to take over all the functions currently performed by the City of Man. On some these functions, like mail delivery or sewage control, the proposition may seem a bit weird, but on others, to take over these functions would be disobedience. The Church does not have the authority of the sword (John 18:36; 2 Cor. 10:4), and should never have the authority of the sword. But the City of Man has a police force, and has this force under the mandate of heaven (Rom. 13:4).
Now this means that if the Church is to be a replacement polis for the City of Man, soup to nuts, then either we make our play now, with a willingness to pick up the sword, or we wait to make our play until we see the Eschaton shimmering on the horizon, and we won’t need a police force. And I suppose we could make our play now, fire all the cops, and enjoy the reign of the saints for the next fifteen days or so until we are all replaced by someone with a brain.
The problem with stepping in now is that we either have to be hopelessly naive, discrediting the gospel thereby, or we have to deny what Scripture teaches us about the Church foreswearing violence. And the problem with waiting until the police are not necessary is that all the texts describing the Church as a new polis are two thousand years old, and have been true this entire time. This is not something that becomes true in the future; it is true now. The Church is the new polis now, and we are commanded to invite all the kings of the earth into it. Now.
Draw a circle on the board and call it the City of Man. At the center of this city is the cultic center, the hall of idols. That is their city, their polis, and they, like all human societies, have a cultus at the center. We in the Church are commanded to preach the gospel to every creature, and as we do this, we are to drive toward that city center, and establish the worship of Jesus Christ there. From that Church center, that new principle of worship, godly influence radiates outward until the kingdom of Man becomes the kingdom of God.
Outside the realm of the Church proper are many human activities that are lawful in themselves, but which are not part of the realm of the Church — auto mechanics, lovemaking, delivering mail, and slaying the wicked. Remember that the Church is called to exercise a strong teaching and disciplinary office, and not a pale saintly influence. The lines pointing out from the new cultus are to be thick, not thin. The Church establishes the Kingdom, is at the heart of the Kingdom, and is the living worship center of the Kingdom. This is necessary because it is certainly lawful and necessary for Christians to put people in jail, but it is not lawful or necessary for the Church to have dungeons. We have done that before, and quite a bit of repenting was necessary — the Church has paid a steep price in history for that kind of overreach. And a bunch of well-intentioned Christians want us to try it again.
But if the Church is merely a subset of the Kingdom, then why is it called a new polis, as if it were the whole thing? The answer is simple — synecdoche, where the part, the most important part, is used as representative of the whole. The worship on Mt. Zion was not all there was to Israel, but it is the most natural thing in the world to speak of the whole thing as Zion. It is the same with the new Zion.