Humanity is divided into two symbolic cities distinguished by what each loves: the earthly city is formed by love of self extending to contempt of God, the heavenly city by love of God extending to contempt of self. The division is not political or ethnic but a matter of the ultimate object of one's desire, so the two cities are intermixed within every visible society.
Worldly success and military triumph do not demonstrate that a people's gods or practices are good, since wicked and virtuous peoples alike experience fortune and disaster. Therefore Rome's past greatness is no argument that its pagan cults protected it, and its recent calamity is no argument that abandoning them caused ruin.
Identical external suffering produces opposite inner results depending on the sufferer's disposition: the same fire that hardens clay melts wax, so adversity purifies the good and corrupts the wicked. The event is therefore not the decisive factor; the prior state of the soul meeting the event is.
Moral quality is assessed not by which objects one loves but by whether one loves them in right proportion and order, loving higher goods more and lower goods less. On this view even the pagans' celebrated virtues, when aimed at glory rather than God, are splendid vices because the love behind them is misdirected.
The members of the heavenly city, while living as pilgrims in earthly society, legitimately use its laws, peace, and institutions as instruments toward their true end rather than treating them as the end itself. This permits full civic participation without final allegiance, since temporal order is a means serving eternal rest.
During the sack of Rome, the invaders uncharacteristically granted refuge in the Christian churches, sparing those who fled there, a mercy war's custom did not require. Augustine uses this to argue that whatever good the Romans received amid catastrophe traced to the faith they blamed, not to the gods they had abandoned.