This is no doubt not only a difficult chapter for us to read but also, I suspect, was a difficult one for Augustine to write.
It would be tempting to read his words sarcastically: "As to those whose hearts... reply that they have never been proud of the virtue of virginity... but condescending to those of low estate, rejoiced with trembling these gifts of God... even such faithful women, I say, must not complain that permission was given to the barbarians so grossly to outrage them." It would be all to easy to read this as Augustine functionally saying "really? You're saying you didn't deserve this, not even a little bit? You, alone among all women of the world are innocent?"
And yet, I don't think that's his intention when we consider the overall arguments he is making, especially in his charge: "faithfully interrogate your own souls." Rome has been sacked, Christian women have been violated, refugees are streaming across the Mediterranean, and Augustine takes the opportunity to remind us that as Christians, there is no situation of Providence in which we find ourselves exempt from self-examination and repentance for our own sin in the sight of a Holy God. The point is not whether or not the women are guilty in this particular instance (he is clear that "I, for my part, do not know your hearts, and therefore I make no accusation; I do not even hear what your hearts answer when you question them"), it is that in every instance we are responsible for continually searching out and putting to death sin in our own lives.
And, when we do so examine ourselves in the direst of circumstances, what we will find is that all Christian virtue flows immediately from the grace of God. "If you did not consent to sin, it was because God added His aid to His grace that it might not be lost."
Our natural temptation is to champion ourselves as bastions of strength and virtue in the face of adversity. Yet the honest Christian, as he matures in the faith and persists in self-examination, is increasingly able to trace the hand of God in his life as that alone which supports and empowers and enables us to live in a difficult world, rather than any reserve of goodness or virtue in us.
Even more, Augustine wants us to see that God is utterly sovereign over all that happens and to be praised by us for it. "For some most flagrant and wicked desires are allowed free play at present by the secret judgment of God, and are reserved to the public and final judgment... As, therefore, some men were removed by death, that no wickedness might change their disposition, so these women were outraged lest prosperity should corrupt their modesty." Even these evils are used by God for the good of His people.
We see here yet another contrast between Christianity and the world--how many other religions can endure war and disease and crime and suffering and still find delight in the presence of God, even the very same God who permits such evils in the first place?
Augustine's conclusion to this chapter is that such things happen for our good, and for God's glory. Whatever does happen to us teaches us to rely ever more upon God and to live in a way pleasing to Him. "...when they settle gain to the firm persuasion that He can in nowise desert those who so serve Him... they are shut up to the conclusion that He could never have permitted these disasters to befall His saints, if by them that saintliness could be destroyed which He Himself had bestowed upon them, and delights to see in them."
God creates "saintliness"; God grows saintliness, often through persecution; and God delights in the finished work of saintliness. As Augustine says at the beginning of the next chapter, "the whole family of God, most high and most true, has therefore a consolation of its own--a consolation which cannot deceive, and which has in it a surer hope than the tottering and falling affairs of earth can afford. They will not refuse the discipline of this temporal life, in which they are schooled for life eternal; nor will they lament their experience of it, for the good things of earth they use as pilgrims who are not detained by them, and its ills either prove or improve them."
Give the truths of the previous chapter, we can clearly see that our God is not answerable to man, and certainly not to men who worship creation. Instead, God is omnipresent, and so not absent when disasters fall and certainly not incapable of stopping them. Rather He sends trials to grow us and, as the Christmas carol says, "fit us for heaven."
When we pare it down to its base elements, we see that the heart of the complaint against Christianity is the desire for luxurious prosperity and self-indulgence. "For why in your calamities do you complain of Christianity, unless because you desire to enjoy your luxurious license unrestrained, and to lead an abandoned and profligate life without the interruption of any uneasiness or disaster?... for your purpose rather is to run riot in an endless variety of sottish pleasures."
Even the pagans know on some level that this is unwise and recognize the tendency in man to incline to idleness and vice when times are prosperous. When conditions are difficult, a state like Rome might be roused in pride to endure hardships and set aside selfishness and pursue a kind of patriotic virtue for the glory of the state. (Leave out for a minute the fact that it's a state which worships demons and idols.) The best pagan minds understood that once those difficult conditions are removed, it's right back to the abject decline of human nature into opulent squalor.
Which circumstance is exactly what we've got today. Christians are attacked because we are seen as standing in the way of the world's happiness, pleasure, and prosperity. How often do we hear those who are openly hostile to the faith decrying those awful Christians who are constantly calling everyone "sinners"; perpetually standing in the way of the social progress that will bless human existence; and always holding on to outdated myths while the rest of civilization has ascended to a higher and more enlightened plane? Those religious nuts are clearly the only ballast holding back a world that would otherwise soar off into the majestic heights of pleasure, wealth, and glory!
Responding to these types is Augustine's main goal, and he does it by pointing out that even the Romans of the past would have agreed with the Christians about the moral decline of the Roman people. (We should note of course that he does not argue for a kind of return to the civil principles of the Roman Founders. Neither should we.) When the Romans were somewhat wiser, they could at least recognize certain forms of sin that should be resisted in the name of the common good, even if they didn't yet know the actual solution to all sin that would be offered in Christ.