Chapters 14 and 15:
These chapters deal with the difficult question of the relationship between Divine power and human virtue. Does God reward the good and punish the evil? If He does not, is that proof that He is a false god and that we should find some other, better, gods? Certainly Christians believe that God does have the power to supernaturally save His followers, as we see from the stories of Jonah and Daniel (Chapter 14). But, in the late sack of Rome, we also see Christians being brutalized as much as the pagans. Maybe then we ought to turn back to pagan gods, who have more to offer us in the way of worldly protection?
The problem with this line of reasoning, argues Augustine, is that it ignores reality. The pagan gods do not protect their own, as we see in the example of Marcus Regulus, who all agree acted virtuously, and who died a most horrendous death at the hands of Rome's enemies. (For other versions of the story, see Tertullian's "To the Martyrs" and Aulus Gellius' Attic Nights). The point of Regulus' example is not that the Christian God would necessarily have saved him, but that even pagans admit that there are reasons beyond worldly health, wealth, and prosperity to worship the gods and live virtuously. And once we've raised our eyes above this world and begun a conversation about higher affairs, the tenor of the our debate changes and we're no longer concerned nearly as much with "who caused the sack of Rome?" as we are with "who is the true God and what is the right way to live virtuously in obedience to Him?" We see then that it is no rebuke to the Christian religion that people suffer--even when those people are the Christians themselves.
Almost in passing, Augustine mentions that we likewise see in the story of Regulus something about the nature of a virtuous city: "But if they say that M. Regulus, even while a prisoner and enduring these bodily torments, might yet enjoy the blessedness of a virtuous soul, then let them recognize that true virtue by which a city may also be blessed. For the blessedness of a community and of an individual flow from the same source; for a community is nothing else than a harmonious collection of individuals."
Since Augustine will have more to say about the nature of a community later in City of God, I won't comment on this now other than to note the parallels with Plato's Republic where the virtue of the individual and virtue of a city are declared to be the same. Not, of course, to argue that salvation comes to a city rather than an individual, Augustine is a Christian after all, but it's still worth noting given how radically individualistic we tend to be in a post-Enlightenment world.