If nothing else, these chapters should help us appreciate Augustine's pastoral concerns. With issues as delicate as rape and suicide, Augustine reminds us that "in discussing it we shall not be so careful to reply to our accusers as to comfort our friends." (Chapter 16) I know my tendency is to dive into the problem of evil with a head-on attack (the shallowness of modern atheists only increases that tendency), while forgetting the people who are actually suffering in the world. We have to love how gentle Augustine is with this, since we know (either from previous exposure to Augustine or just from knowing that he's an orthodox Christian) that he will eventually take the point that all the evil which stains our soul comes from within us. Augustine's delicacy does not come from a wish to soft-peddle evil, but rather from a desire to care for those who have been hurt.
In this instance, Augustine is clear that virgins who have been raped during the sack of Rome are not guilty because of the rape, and that we should at least sympathize with those who are driven to suicide in defense of their virtue. These topics require a tenderness that I frankly lack in my day-to-day life, and that we rarely see even from the most famous of Christian leaders and thinkers. (Imagine, for instance, what a Tertullian or a Luther would say in such an instance!)
So I think if we lay out the brief versions of Augustine's arguments, we can see what he's getting at.
-Being raped is not a sin.The overall point is that we can be honest about the nature of sin, sympathize with a sinner pursuing virtue in a sinful way, and condemn the sin itself, all at the same time. (And that convoluted run-on sentence is why Augustine is Augustine and I am not.)
-Wanting to commit suicide to preserve moral purity is a circumstance that merits sympathy and care.
-Committing suicide is a sin.
-Sin itself is found in our desires, not in our bodies.
In being honest about the nature of sin, we need to remember that it is found in the inclination of our souls as expressed through our will. "Suppose a virgin violates the oath she has sworn to God, and goes to meet her seducer with the intention of yielding to him, shall we say that as she goes she is possessed even of bodily sanctity, when already she has lost and destroyed the sanctity of soul which sanctifies the body?" Or, in a more abstract example, imagine Eve was struck by lightening just as she was reaching for the forbidden fruit--wouldn't we all agree that the fall had occurred even though no fruit had been eaten?
This is not to say that the body is irrelevant (again, Augustine is no Gnostic); but it is to say that what happens to the body is not ultimate. Instead, we must look at how we use our bodies and trace those actions back to our souls. It is not a sin to be hit by a car--a traffic accident is no indication that we have become more depraved. But it is a sin to step into traffic with the intention of ending our own lives. Both are actions we do with our bodies, but the virtue or vice of each action is determined by the intent with which we take it.
Again, I think that Augustine's application of these points is pastoral rather than apologetic. He's equipping Christians to think carefully about the nature of sin, even as we care for those who are suffering from it (whether their own or someone else's). In light of his previous charge not to be negligent in condemning sin, this passage is critical in tempering our thoughts and words and in keeping us from a rigid fundamentalism that would ignore the necessary subtleties that come from living in a complex world.
With that said, he's opening the door to something much more offensive to the world than the condemnations of things we can all agree on. Presumably everyone (Christian, pagan, atheist, etc) can agree that women who have been raped are not guilty of evil, and that evil has been done to them. The rapist is guilty, the victim is not. That's the easy and unoffensive bit. But when Augustine says that purity "belongs to the soul" and that "not even when the body is violated is it lost," he's setting up the Christian doctrine of indwelling sin. We are not guilty when something awful happens to us, but we are darn sure guilty when we do something awful. Again, pastorally this is not the place to have that discussion, but we can see it being set up for the future.