Augustine is branching out from his descriptions of Roman and Greek theatre to a broader conversation about Roman culture. The Roman gods and religious practices, the Roman theatre, and Roman history all contributed to making Roman moral culture. The gods are immoral, as we’ve seen, and Roman drama and comedy enjoyed putting their immorality onstage. Roman history – the rape of the Sabine women and the golden ages of Roman morality – shows that Roman culture has always been immoral, even at its best. The one era to which Augustine can point as a time when the Romans lived morally was made by the threat from Carthage – Rome was incapable of making a moral culture for itself. Under threat, the Romans constrained themselves, but in times of success and wealth, they were depraved. This, according to Rome’s own historians.
It looks to me like Augustine is writing an alternate story of Rome, one centered on its moral culture instead of its military and political successes. Augustine clearly sees the world differently than his opponents do. He sees the quality of a people as defined by its moral character and the truth of its worship and not by its power or wealth. In this alternate story, Rome has always been an unsuccessful nation because of its consistent immorality. Augustine’s opponents are wrong to long for an old age of Roman success because any successes were built on a false foundation; immoral and unjust Rome could not last.
At the end of chapter 18, Augustine points to Christ as the builder of a city with a true and just foundation. The City of God is actually eternal and legitimately strong, unlike the illusion of eternity to which Rome pretended. Christ as king makes a moral and just people, whose character may not be fully revealed among the city of man but puts its citizens to shame. Augustine, at the end of chapter 18:
“[B]y his most wholesome doctrine Christ forbids the worship of false and deceitful gods; and by His divine authority He detests and condemns the poisonous and shameful lusts of mankind. Indeed, He is by degrees withdrawing His servants from a world decaying and collapsing under these evils, in order to build with them an eternal and most glorious City: a City founded not upon the plaudits of vanity, but on the judgment of truth.”