It’s common to associate Augustine’s Two Cities with Luther’s Two Kingdoms. But they are really quite different. In The City of God, Augustine defines the two in terms of two different loves: The City of God has to do with the love of God; the City of Man has to do with love of self.
Thus the two cities are in opposition to each other. This is a scheme for dualism, for ascetic rejection of the world, giving rise to monasticism.
Luther’s Two Kingdoms is a paradigm for embracing the world. The Kingdom of the Left, for Luther, is about neither love of God nor love of self, but love of neighbor.
To be sure, Augustine’s distinction points to a reality, the conflict Christians have in living among non-Christians. So his work has much to offer. Luther recognizes the Christian’s conflict with the non-believing world, the realm of worldliness.