As you know, The City of God is an enormous work. Therefore, many things can be said. In light of this, let me name a few of them.
First, it is clear that Augustine believes in the justice of God. Therefore, God's justice must be in cities as well. Moreover, citizens should be ethical and upright citizen of cities. In this regard, Christianity is a force for civic virtue, justice, and order.
“Justice being taken away, then, what are kingdoms but great robberies? For what are robberies themselves, but little kingdoms?”
Second, in the first half of the work, Augustine argues that it was not Christianity that caused the sack of Rome in 410. In the past Rome had many disasters before Christianity. More to the point, the Christian God was actually favoring Rome in the past on account of virtuous people. This is why it lasted so long. Therefore, Christians, who worship this God, actually benefit the cities of this world.
Finally, if we take Augustine's two kingdom framework and tease it out, there is a very good application for civic involvement. Because Christians are citizens of the heavenly kingdom, they can be the very best citizens on earth. In other words, heavenly security allows Christians to serve people here with generosity.