Saint Augustine’s two cities are ultimately mystical in nature, each containing a visible component among the living generation struggling in this world for either material or spiritual goods. Those striving for the city of God live in, but are not of, the city of man, seeking rather to transcend the material order by living in obedience to God’s commands.
In Augustine’s view, the Church, though amid the city of man, serves as the visible sign of God’s love among the living and as a means of grace to strengthen its members’ life of charity and virtue. It teaches obedience to the moral law and offers sacraments of life-giving grace as it seeks to advance the city of God, calling all people to obedience and away from sin. It cooperates with the city of man to advance peace through justice in human affairs. It urges the punishment of evil, even to the point of just war, in order to preserve and protect the innocent and establish a just and fruitful peace. It serves as a sign of God’s mercy, offering sinners a haven from a world too often mired in violence and selfish disregard of the common good.
Augustine’s ultimate message is that Christ died for the redemption of this sad and sinful world, and his resurrection points to the final judgment, in which God will separate the two cities. Those proud citizens of the city of man will suffer eternal separation from God, in the second death of hell, while the sojourners who loved and obeyed God on pilgrimage in the city of man will be rewarded with eternal bliss.