Augustine here argues for a realistic view of apologetics. We certainly need to be able to respond to the challenges of the world, but we also need to realize that the world will not hear us apart from Divine grace. And so we need to be thoughtful and balanced in how we try to defend the faith before a blind and obstinate world. In the 20th and 21st centuries, there has been a strain of apologetics which seems to believe that if only one piles up fact after fact and argument after argument, then the hearer will have no choice but to bow under the mountain of evidence and turn to the faith. (Obviously, there is some place for the work that goes into this approach!) Augustine, however, would have us remember that no one is argued into being a Christian, and that our responsibility is to be faithful in sharing and living the Gospel, not to exhaust ourselves in responding to every rebellious claim of a stony heart.
In the summary of Book I, we are reminded that in the moral ruin of the world, the only true shelter of any kind comes through Christ. And because of that, Christians should take comfort, even when great wrongs are done to them.
Some have argued that Christianity is the cause of the evils of the world. This just shows either 1) their ignorance of history (as with the masses) or 2) their dishonesty (as with the elites, who do know their history but keep it quite out of prejudice). In reality, bad things happen at all times and all places--including in Rome before Christianity came along.
We of course see the same sorts of arguments made today: "If only those backwards Christians would get out of our way, our scientific progress/moral development/public policy would shoot ahead and we would live in an opulent paradise!" (We Christians of course are not innocent of this kind of thought.) What we have to remember is that Providence governs all, and that even in a society completely without Christians disaster and calamity occur.
"By their fruits you shall know them" What moral standard does your god reveal? What kind of behavior does He delight in? If his followers revel in awfulness and say that it is pleasing to their gods, then we can be suspect about whether or not the 'god' in question is one or not. And we can certainly be suspect concerning the charges leveled by the follower of that God against Christians.