A multitude of gods does the Romans no good at all. The Romans were famously syncretistic, adopting nearly all of the religions they encountered (with a few major exceptions, namely the Bacchae, the Druids, and after a period of toleration, the Jews and the Christians). And yet despite the plethora of deities, at no point in their history could they said to be safe under the protection of any of them.
I don't know if the parallel is intentional or not (and I'm too lazy to look up the Latin original), but at least in the Dodds translation Augustine talks about the city of Rome as "enjoying the protection of such a cloud of deities," and that so it "might surely have been preserved from some of those great and horrible calamities" if those deities had any power at all. This makes us think of the passage in Hebrews that describes the Christian faith: "Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God." There are numerous contrasts which we might mention: the Christians have only one God, but He has already won the race; the pagans are surrounded by a cloud of impotent beings, while the Christian runs surrounded by brothers and sisters in the faith; the pagans are surrounded by sinful beings, while for the Christian the act of running towards Christ results in the falling off of sin.
Again, I don't know if the parallel is intentional or not, but it's at least worth noting, so long as we're careful not to slip the cloud of Christian witnesses into the place of the pagan gods--Augustine would be horrified by that (to say nothing of the author of Hebrews!).
Lest we be misled by Augustine's closing statement in the previous chapter, he goes on to remind us that even in its youth when it had few gods, Rome was by no means virtuous. Rape, fratricide, and all manner of sin defined even the youth of the state. "See what rights of marriage these were that fomented unnatural wars. These were the Roman leagues of kindred, relationship, alliance, religion. This was the life of the city so abundantly protected by the gods."
Whatever we think of the present (and Augustine certainly has much to say about that!) we should not be tempted by the idea of a past 'golden age' to which we should try to return. While to be sure the Christianity looks in faith to a one-time occurrence in the past as its foundation, in terms of human history and the city of man it is exclusively future-oriented. We do not strive in this world to restore the lost golden age of the Roman (or American) founding, we look instead to the future return of Christ when the world will be judged and made new, and all that is wrong set right. Part of keeping that good future in view involves having a realistic view of the sinfulness of the human past (and present).