In book 20, Augustine details the final judgment. A few quotes, and then some comments:

Augustine says that God’s judgments are mysterious and beyond our understanding. What do we do then?

“We do not know, therefore, by what judgment God causes or allows these things to come to pass; for in Him there is the highest power, the highest wisdom and the highest justice, and in Him there is no infirmity, no rashness, and no injustice. For all that, it is salutary for to learn not to attach great value to those things which, whether good or evil, we see to be common to good and evil men alike; but to seek instead those things which belong only to good men, and especially to shun those evils which belong only to evil men. However, when we arrive at the day of judgment … it will become apparent that God’s judgments are entirely just: not only all the judgments that will be given then, but also all the judgments given since the beginning, and all those which are to be given hereafter until that time. In that day, too, it will be made manifest by what just judgment of God it come about that at this present time so many – indeed, almost all – of the just judgments of God are hidden from the sense and minds of mortals. However, in this matter one thing is not hidden from the faith of the godly; and that is, that what is hidden is nonetheless just.”

Sin has infected all humanity, and we all are powerless to escape it. What hope do we have, then? Our hope is Christ, who is alive but died for us, that we might be raised with Him:

“All men are dead in sin, then, and no one at all is exempt, whether in original sin or intentional sin added to it, committed either in ignorance or by failing to do what is known to be right. And for all the dead, there died the one Man Who was truly alive: the one Man, that is, Who was entirely without sin. He died so that those who are brought to life through the remission of their sins should henceforth live not for themselves, but for Him: for Him Who died for all, for our sins, and rose again for our justification. He died so that we, believing in Him who justifies the ungodly, and being justified from ungodliness by Him and raised from the dead, might be able to share in the first resurrection that ‘now is’. For only those take part in this first resurrection who are also to be blessed for all eternity.”

Most of the rest of the book is a fairly detailed account of Revelation: the martyrs (he says they represent the whole Church), the beast (who will persecute the Church), Gog and Magog (those people in whom Satan dwells), the thousand year reign (not, according to Augustine, representing eternal bliss), and the coming of the New Jerusalem. [Josh, take note: this is a helpful discussion for thinking about Revelation and a very specific answer to one of your early questions. Yes, Augustine explicitly has Revelation in mind as he writes this book.] One quote on the New Jerusalem:

“This City has been coming down out of heaven since its beginning, from the time when the number of its citizens began to increase in this present age by the grace of God which comes from above through the ‘washing of regeneration’ in the Holy Spirit sent down out of heaven.”

The last several chapters of book 20 look at Old Testament prophecy and explore how it will be fulfilled in the New Jerusalem. Interesting.

In sum, book 20 looks from a few different angles at the final judgment. Augustine argues that it will happen, that it has not yet happened, that justice will be clearly made known, that life and resurrection will have final victory over death and sin, and that the City of God that has been invading creation for many generations will complete its invasion and fulfill God’s purposes for what He made. Overall, I don’t have much to add. A few quibbles with a few details of Augustine’s reading of¬†Revelation, but I can deal with that in my work on Revelation.