16 centuries ago, the greatest empire the world had ever seen, The Roman Empire, symbolically, with the sack of its capitol, the eternal city of Rome, died. It was the end of 1,000 years of civilization by the hands of barbarian hordes from Germania. Many Romans argued at the time that the Empire was crumbling because the ancient gods had been displaced by the now official-and dominant-Christian faith.
The City of God is a rebuttal of this belief-but that is far from all that it is. In addition to being an apologetic work of Christian influence, is also one the most important theological constructions on theological anthropology (esp. human depravity, political instability) and the glory of God, of his kingdom, his 'City' his righteousness. Moreover, the book is perhaps the most important and groundbreaking political philosophy the church has yet to produce.
Second only to Augustine's Confessions in popularity, and spreading a wider theological net than his De Trinitate, The City of God is the greatest and most comprehensive of Augustine's works. It is impossible to understand his thought, or its trajectory without this work.