Excerpts from Books and Wikipedia
"He was a Latin philosopher and theologian from the Africa Province of the Roman Empire and is generally considered as one of the greatest Christian thinkers of all times. . . . In his early years he was heavily influenced by Manichaeism and afterward by the Neo-Platonism of Plotinus. . . . He believed that the grace of Christ was indispensable to human freedom, and he framed the concepts of original sin and just war. When the Western Roman Empire was starting to disintegrate, Augustine developed the concept of the Catholic Church as a spiritual City of God . . . His thoughts profoundly influenced the medieval worldview." [Wikipedia] "Rome, he wrote, was a city of man; and in all times, at all places, the cities of men exist side by side with the city of God, the true Eternal City, the unseen spiritual kingdom. Men choose which city they will occupy, and although the goals of the two cities may occasionally intersect (and their citizens may find themselves able to cooperate with each other), the ultimate purposes of their citizens diverge. The city of man seeks power; the citizens of God’s city seek only the worship and glory of God." [Bauer: Medieval World, p. 77-84]