I promise nothing systematic, structured, or even necessarily coherent.
A few notes from the Preface:
1) The city of God "lives by faith in this fleeting course of time." This of course points us to Hebrews 11, where we see that the source of Christian life is not ourselves or our works, but is our faith in Jesus Christ. As Americans we're used to thinking of this "faith" as an individual thing--and this is not necessarily completely wrong, since salvation is an affair for the individual alone. Augustine, however, reminds us that faith is also that which undergirds the city of God's people. I am saved by my own faith and not that one someone else, but the whole city of God into which I am drawn is a faith-based community (and I don't mean that in the empty way it's thrown around these days). Where the city of man is built upon lust and arrogance, the city of God exists by faith.
2) The city of God is founded upon grace, rather than upon human works. We see this when Augustine says that the distinctive mark of the city of God is humility, "which raises us, not by a quite human arrogance, but by a divine grace, above all earthly dignities that totter on this shifting scene." One visible characteristic of the city of God is humility, but this humility is founded not on human ability or righteousness (which would then be a matter of pride), but upon grace.
When we look for the city of God, we should spot it by its humility, but our glance should never stop at humility but should instead pass through that humility to the foundation of Divine grace upon which it is constructed. In other words, looking at a Christian or at a gathering of Christians should force the eyes of the observer to shift upwards to Christ.
3) The city of God is a pilgrim city, passing through this world as a "stranger in the midst of the ungodly." This carries with it two sub-points:
a) The city of God has a destination that is not a part of this world. We must never look for our ultimate fulfillment on earth, but must keep our eyes focused on our destination "in the fixed stability of the eternal seat," where we shall have "final victory and peace."Chapter 1:
b) Lest this cause us to declare the world a "lost cause" and withdraw into splendid Christian isolation (a horrible thought for anyone who believes in indwelling sin!), we also see here that for now the city sojourns in the world. It exists "in the midst of the ungodly", so much so that when we speak of the city of God, we "must speak also of the earthly city." Reflecting on the salvation that comes by grace through Christ requires us to remember our sin from which we have been saved, and thinking about the city of God requires us to think about the city of man where it temporarily dwells. At no point does Augustine give us permission to wash our hands of worldly affairs and withdraw into Christian seclusion.
1) The goodness of God to His people spills over onto the world, so that the whole world benefits from it. The city of man exists in the shelter of the city of God, just as the pagans who fled to Christian temples were spared because of the peace and security found there.
2) Such is the nature of sin that even when this mercy is clear and unobscured, when all can directly see the kindness of God to all mankind, the city of man persists in its rebellion. Augustine tells us that those who "ought... to give God thanks, and with sincere confession flee for refuge to his name" instead "in ungrateful pride and most impious madness... perversely oppose that name under which they fraudulently protected themselves for the sake of enjoying the light of this brief life."
The fact that so many were saved by fleeing to even the mere church buildings (and I've read enough Augustine to know that he doesn't confuse the building with the city of God) shows that there is something unique about the city of God. The normal order of the world is that the powerful crush the weak while their stand by impotently. This was true before the rise of Rome and is the way of the world now. There can be no salvation or safety under the pagan gods.
Just for Fun:
An Augustinian hymn:
Words and Music by Stuart Townend, Keith Getty, Kristyn Getty
By faith we see the hand of God
In the light of creation's grand design
In the lives of those who prove His faithfulness
Who walk by faith and not by sight.
By faith our fathers roamed the earth
With the power of His promise in their hearts
Of a holy city built by God's own hand,
A place where peace and justice reign.
We will stand as children of the promise,
We will fix our eyes on Him, our soul's reward;
Till the race is finished and the work is done,
We'll walk by faith and not by sight.
By faith the prophets saw a day
When the longed-for Messiah would appeal,
With the power to break the chains of sin and death
And rise triumphant from the grave.
By faith the church was called to go
In the power of the Spirit to the lost,
To deliver captives and to preach good news,
In every corner of the earth.
By faith this mountain shall be moved
And the power of the Gospel shall prevail,
For we know in Christ all things are possible,
For all who call upon His name.