The City Of God was written around 420 A.D. in response to the sacking of Rome by the Visigoths in 410 A.D . Many Romans believed that the sacking of Rome occurred because the pagan Roman gods were angry with Romans for abandoning them in favor of Christianity. Saint Augustine combated this by effectively saying that Rome, because it is an earthly city, does not matter; only the city of God matters. According to Saint Augustine the “city of God” is filled with believers while the earthly city is filled with nonbelievers. This division allows Saint Augustine to argue that the church is part of the city of God, but the city of Rome is earthly and thus expendable, and because the city of God (the church) is intangible it is indestructible.
Saint Augustine argues this idea by stating the difference between the two cities is in the goals of its inhabitants. According to Saint Augustine the inhabitants of the earthly city seek physical and financial wellbeing with their only goals being peace in wealth. (Just to stick my two sense in isn’t that a very good goal?) However, the inhabitance of the city of God do not seek peace but instead use peace to further the city of God. These people do not look for earthly peace or wealth, but rather look to the next life as their goal.
In general he says that the two cities are part of each other, but that the differences between them deal mainly with life goals and the important things in life.