Written in the 5th century CE, City of God, or The City of God Against the Pagans, is one of the best known and most influential of Saint Augustine’s works. The book was completed less than two decades after the sack of Rome by the Visigoths in 410, a historical context extremely important to Augustine’s claims.
City of God is influenced heavily by Platonic and neoplatonic ideas, especially that of the existence of universals: ideal forms which exist solely as ideas. All Earthly things are imperfect replications of such ideal forms: Rome, at the time pervaded with pagan ideas and pagan worship, was what Augustine called the “Earthly City," an imperfect representation of the ideal “Heavenly City” or “City of God” where religious devotion and morality prevailed.
While City of God’s primary focus is on the Earthly City vs. the City of God, Augustine’s ideas about other hotly debated religious ideas--such as original sin, just war and free will--are also discussed throughout the texts. Augustine’s ideas and theological contributions earned him the titles of Church Father, Doctor of the Church, and, of course, Saint.