Throughout the Republican period, war, civil discord, and natural disaster was not only common, but functionally unceasing. It's hardly fair, then, to blame Christ or the Christians for the terrible circumstances facing the Romans. We've seen all of this many times (and no doubt will hear it again).
But there's something new here that's interesting. In the course of his exposition, Augustine drops a hint about the method Christians ought to use when engaging the world. Having just cited the pagan historian Sallust on the calamities that have befallen the Romans, Augustine tells us:
Now, if those historians judged than an honorable freedom of speech required that they should not be silent regarding the blemishes of their own state, which they have in many places loudly applauded in their ignorance of that other and true city in which citizenship is an everlasting dignity; what does it become to us to do, whose liberty ought to be so much greater, as our hope in God is better and more assured, when they impute to our Christ the calamities of this age, in order that men of the less instructed and weaker sort may be alienated from that city in which alone eternal and blessed life can be enjoyed? Nor do we utter against their gods anything more horrible than their own authors do, whom they read and circulate. For, indeed, all that we have said we have derived from the, and there is much to say of a worse kind which we are unable to say.
I think there are three things we can take away here:
1) Whenever possible, we should draw facts from worldly sources.This is of course not to say that Scripture is not sufficient for the life of a Christian--heaven forbid that we ever say that! But, when talking to non-Christians, whenever possible we ought to argue with them from their point of view. While we certainly should argue about, for example, original sin from the pages of Scripture, we should also point to worldly historians, news stories, and other sources which non-Christians will be hard pressed to deny are examples of naked human evil. This further means that we need to be aware of such sources and competent to speak from them, as Augustine clearly is with Sallust, Cicero, Virgil, etc.
2) Whenever possible, we should draw criticisms from worldly sources.
3) We must speak freely in unbending honesty.
Further, when doing this we need to be relentless in our analysis of the dire situation the world is in. Those outside of Christ are on the direct path to hell as even their own writers admit, and anything less than open honesty is doing them a disservice. While of course in personal settings gentleness and delicacy may be called for (something I certainly need to work on), in generally honesty about sin is always the best policy.