The biggest reason that I took on the immense challenge of making it through this work is "perspective". Rome was sacked by the Visigoths in 410, Augustine began this work 3 years later in 413 and did not complete it until 426.
Rome had BEEN "civilization" for a thousand years prior, and naturally in 410, St Augustine and his peers believed they were living in "modern times", all be it a time of great change and disruption at the ending of a thousand-year reign which they had assumed would last forever.The work is remarkably lengthy and wordy (867 rather small type pages in my copy) and decidedly NOT an "easy read". I must say though that the sheer volume and many asides and references to other scholars of the day give an insight into the intellectual life of the very elite of that day that feels important in a way that is hard to express. Perhaps the difference between walking across the US vs flying over it in a jet?
I will include this one rather lengthy quote as an example of the style and the fact of "every age believes they are modern" ... and highly superior to those that have gone before. Note the reference to "less educated ages", but interestingly from the perspective of "only 600 years"! How much more arrogant we have become in our day -- we are nearing the 500th anniversary of the Reformation in 2017, yet it is hard to imagine someone asserting ONLY 500 years! Today, the year 2000 seems "ages ago" to many of our clickbait attention spans.
It is most worthy of remark in Romulus, that other men who are said to have become gods lived in less educated ages, when there was a greater propensity to the fabulous, and when the uninstructed were easily persuaded to believe anything. But the age of Romulus was barely six hundred years ago, and already literature and science had dispelled the beliefs that attach to an uncultured age. And a little after he says of the same Romulus words to this effect: From this we may perceive that Homer had flourished long before Romulus, and that there was now so much learning in individuals, and so generally diffused an enlightenment, that scarcely any room was left for fable. For antiquity admitted fables, and sometimes even very clumsy ones; but this age [of Romulus] was sufficiently enlightened to reject whatever had not the air of truth. Thus, one of the most learned men, and certainly the most eloquent, M. Tullius Cicero, says that it is surprising that the divinity of Romulus was believed in, because the times were already so enlightened that they would not accept a fabulous fiction. But who believed that Romulus was a god except Rome, which was itself small and in its infancy?The work starts with a lengthy defense of Christianity against the charge made by many in that day that failure to pray to the gods of Rome due to the conversion to Christianity was the cause of the city being sacked. It then discusses the "City of God" -- the Church, vs "The City of Man". Secular government all in MUCH detail, with references to Plato and other Greek thought which start The Church on a path of melding Greek Philosophy (especially Plato) and reason into Christian theology. This "Hellenization" of Christianity is the major historical effect of this work.
At its simplest, it is the story of the city of man -- selfish, mistaking means with ends, worshiping the temporal, attempting to glorify the profane physical human. The story of war, death, destruction and eventually eternal pain.
And of the City of God -- selfless and caring, realizing that the end is pre-ordained and guaranteed by the blood of Christ (the 2nd Adam) to be perfect. Glorifying only God. The story of Grace, Peace, Faith, Love slowly traveling in a path known only to God to perfect union, Love and bliss for all Eternity.
It is not a book that I would necessarily recommend for most -- it is CERTAINLY not "efficient", and one would be well served by skimming and focusing on key chapters -- say "books" 14, 19 and 22. If you desire a worthy challenge however, and want to be rather humbled by perspective, I do believe that you will find yourself rewarded!