What follows is my summary of the abbreviated version of Gilson’s introductory essay that appears in the Image edition of the City of God that I’m reading. It runs on a bit, so I’ll put my commentary in a separate post.
 
Gilson emphasizes that our modern aspiration to build a perfect, universal society had its origins in Augustine’s description of the City of God and the way this City cooperates with, and shares benefits with, the City of Man. However, the modern world neglects a fact that the ancient world would have found impossible to deny – that every society is held together (i.e., merits the name of “society”) only insofar as it is united by two things: religion and blood kinship. He goes on to say that Augustine, in writing The City of God, demonstrates that the City of God, or the Heavenly Society, also is defined by these two factors – religion, in that it consists only of those who have held God as their highest good, and kinship, in that it comprises those who recognize not only the physical brotherhood of Men (all descended from the same original parents) but, more importantly, the spiritual brotherhood of Man (adopted sons of God the Father, through His divine Son, Jesus Christ).
 
In Gilson’s account, Augustine demonstrates that Rome, long before his own day, had ceased to merit the name civitas (“society” or “City”) in the sense Augustine uses the term. That is, it is no longer bound together by a concern for the common good, or a shared understanding of what that good is. Going farther, Augustine demonstrates that the earthly city (not merely the city of Rome, which is its concrete exemplar) has been at odds with the heavenly society since the first generation of mankind, when Cain, who was motivated by pride and self-interest, killed his brother Abel, who worthily worshipped God; thus Augustine illustrates the difference between these two societies, which is the difference between their two primary loves (God or self). By defining things in this way, Gilson shows that, on Augustine’s terms, no earthly city can, with perfect justice, claim the name of “society,” because its primary motivation will always be self-interest (in early Rome, this meant honor or public recognition, later wealth, power, and pleasure) rather than Charity.

Thus, suggests Gilson, any later generation which is inspired by The City of God to create a just, unified, and peaceful society needs to recognize that such a society must be founded on a love of God and neighbor and depend on the grace of God for its peace and unity, and that the any efforts in this world will always, necessarily, fall short of the perfection that can be known only in the life of the world to come. This is why modern efforts to create worldwide social unity are doomed to fail, because, as Gilson puts it, “they have studied everything except the Christian faith in order to find a common bond.” He suggests, “If we really want one world, we must have one Church, and the only Church that is one is the Catholic Church.”