According to Augustine, the distinction between the two cities — the city of God and the city of man — is grounded in the two loves: love of God and love of self. The former leads to genuine fellowship and a communion of mutual giving and receiving, while the latter engenders strife, war, and the desire to exercise domination over others.
Ultimately, Augustine says, these two loves and two cities are themselves grounded in God’s eternal predestination. Although the city of man is destined to perish, God is both creating a new city (the church) from its ruins and preserving the old city by His common grace until ultimate peace and justice arrive with Christ’s return. In this era of common grace, God “sends rain on the just and on the unjust” and calls us to imitate His clemency (Matt. 5:43–48). So Christians have two callings: the high calling in Christ to belong to His body and the calling to the world as citizens, parents, children, friends, coworkers, and neighbors. Because God is still faithful to His creation, there is the possibility of an earthly city with its relative peace and justice; because God is faithful to His electing purposes, there is a church in all times and places that brings true peace and justice. He does this first of all by uniting sinners to Christ, and then one day by eradicating all strife from the earth at Christ’s return.
Consequently, each city has its own polity, serving distinct ends through distinct means. Although some of its citizens are converted to citizenship in the city of God, the earthly city is always Babylon. Like Daniel, believers pray for the city, work in the city, contribute to the city’s general welfare, and even fight in its armies. However, they never forget that they are exiles and pilgrims. Babylon is never the promised land.