For the intelligent, the previous books will have been enough to convince them that Augustine was right about everything. But because there may still be a few dullards out there reading, and because the issue is of eternal importance, Augustine has to keep writing.
Okay, maybe that's not exactly what Augustine meant, but I suspect it's close to what he wanted to say...
Even if the "Civil" gods (and hence also the "Mythical" ones) are rejected as a pantheon, it may be possible that one or two of them stands as a real god amongst the many false deities. We can't just dismiss this claim with a joke as Tertullian did, we have to take this seriously.
Varro highlights twenty gods out of the pantheon as real and worthy gods. But what, Augustine asks, makes one of the pantheon more worthy than the others? Is it their supposed function? Or the number of people dedicated to them?
It cannot be the former (Augustine does not take up the latter in this chapter), for if some gods are exalted over others because of their function, we have a serious problem when we consider all the functions of the deities who have been passed over. If we select Venus and Liber (who govern conception) but pass over Vitumnus and Sentinus (who give life to the fetus), we can't hope to have a viable embryo, however much attention is paid to the gods of the conception.