This earth, this world is our Father’s world. Jesus has bought it with his blood and has taken up the mantle of Adam. He has fulfilled the covenant of obedience in perfect detail. He will continue to rule and to reign until all things are beneath his feet. He is ruling and reigning right now. And when they saw him, they worshipped him; but some doubted.
Augustine’s two kingdoms were the city of God and the city of man. They overlap but both are under God in the juridical sense. There is no God free zone in the Christian religion. Nor in the world of men.
The Bible’s two kingdoms are the kingdom of God and the kingdom of the devil. Neither is equated with the state or any earthly institution.
In any case, they are certainly not the church and the state. The church is God’s church and every state is God’s state.
And this does touch on “Christian Nationalism”, which is intended as a trap made of words but there is some unavoidable Christian truth in there. Which is the nation that the Christian would not have a “Christian Nation”? One in which Christ is respected and exercises cultural authority? One in which the streets are safe and the children can play outside? Where the women can go about unmolested and the police do their duty. Where the courts convict the guilty and set free the innocent? Where true religion flourishes and evil is blunted by the freedom of the preached word?
That the United States be a Christian nation is small potatoes in the context of our God’s eternal plan and Jesus is a very intentional monarch. Nothing less than all will satisfy his global ambition.
In one sense, all of the nations are Christian nations because Christ owns them all. In another sense none of them are because they drown themselves in ungodliness without correction or repentance.