The first ten books of The City of God, which make up the first part of the work, refute the pagans’ charges that Christians brought about the fall of Rome. The first five books deal with the pagan belief that people must worship the old gods to achieve material advantages in this world, including the continuation of the Roman Empire and the supremacy of the city of Rome. In Book I, Augustine attacks the pagans, who claimed that Rome fell because the Christian religion had weakened it, and he stresses that misfortune happens to everyone. In Book II, he demonstrates that the fall of Rome is not a unique event in human history. The Romans suffered calamities before, even when the old gods were being actively worshipped, and those gods did nothing to prevent those calamities from happening. He suggests Romans became weak because of these gods, since they gave themselves up to moral and spiritual corruption. In Book III, Augustine continues discussing catastrophes that occurred in pagan times to further prove that Christianity did not cause Rome to fall. To drive home his point, he asks again why the old gods did not defend Rome in the past.
In Book IV, Augustine suggests an alternative view. Rome endured for many centuries because it was the will of the true God, and its survival had nothing to do with pagan gods such as Jove, who behaved only in the lowest manner. In Book V Augustine addresses the pagan notion of fate, which many people saw as a viable force that had held the Roman Empire together. Rather, says Augustine, the Romans of ancient times were virtuous, and God rewarded that virtue, even though they did not worship him. When he reaches Book VI, Augustine shifts focus and devotes the next five books to refuting those who said people must worship the old gods to gain eternal life. Augustine uses pagan authors to destroy this notion by saying that the gods were never held in high regard and so all the old ways, old myths, and old laws are useless in ensuring eternal happiness. This piecemeal destruction of pagan theology continues through Book X.
Book XI begins the second part of The City of God, where Augustine describes the doctrine of the two cities, one earthly and one heavenly. In the next three books he details how these two cities came about, based on his reading of the Bible. The next four books explain the prehistory of the city of heaven, from Genesis to the age of Solomon, whose story is allegorized as Christ and the church. In Book XVIII, Augustine undertakes a similar process of portraying the prehistory of the city of the world, from Abraham to the Old Testament prophets. Augustine focuses on how the two cities will end in Book XIX, and in the process he outlines the nature of the supreme good. He emphasizes the idea that the peace and happiness found in the heavenly city can also be experienced here on earth. Book XX deals with the Last Judgment and the evidence found for it in the Bible. Augustine continues with this theme in Book XXI and describes the eternal punishment of the damned, arguing that it is not a myth. The final book, Book XXII, tells of the end of the city of God, after which the saved will be given eternal happiness and will become immortal.
Augustine created a theology of the self in Confessions, and in The City of God he initiates a theology of history. He uncovers a wide-ranging explanation of history that begins with creation itself, moves through the turmoil and upheaval of man-made states (the City of the World), and continues to the realization of the kingdom of God (the City of God). In effect, The City of God is a completion of the project he began in Confessions, where he traced the progress of the self toward completion in God. Likewise, human society finds completion in the realm of God. Along with a theology of history, Augustine seeks to put together a Christian philosophy of society. In other words, he gives the various areas of philosophical inquiry, such as ethics and politics, a unity in the universality of divine revelation. History completes itself in divine law. The philosophers of the past, such as Plato, had all said that a person does not owe full and absolute loyalty to any earthly society, and Augustine rigorously critiques this concept in the light of Christian doctrine. He states that the Scriptures alone can instruct human beings about the highest good and the highest evil and that without this guidance, human endeavor has no purpose.
Augustine presents the four essential elements of his philosophy in The City of God: the church, the state, the City of Heaven, and the City of the World. The church is divinely established and leads humankind to eternal goodness, which is God. The state adheres to the virtues of politics and of the mind, formulating a political community. Both of these societies are visible and seek to do good. Mirroring these are two invisible societies: the City of Heaven, for those predestined for salvation, and the City of the World, for those given eternal damnation. This grand design allows Augustine to elaborate his theory of justice, which he says issues from the proper and just sharing of those things necessary for life, just as God freely distributes air, water, and light. Humankind must therefore pursue the City of Heaven to maintain a proper sense of order, which in turn leads to true peace.
In effect, The City of God is a challenge to human society to choose which city it wishes to be a part of, and Augustine sees his task as clearly marking out the parameters of each choice. Augustine concludes that the purpose of history is to show the unfolding of God’s plan, which involves fostering the City of Heaven and filling it with worthy citizens. For this purpose, God initiated all of creation itself. In such a grand plan, the fall of Rome is insignificant.