Scripture tells us that Cain founded a City, whereas Abel, as a pilgrim, did not found one. For the City of the saints is up above, although it produces citizens here below, and in their persons the City is on pilgrimage until the time of its kingdom comes. At that time it will assemble all those citizens as they rise again in their bodies; and then they will be given the promised kingdom, where with their Prince, ‘the king of ages’, they will reign, world without end.
[Augustine, The City of God, 15.1]
In case one begins to think that Augustine’s two cities operate simultaneously in the same manner, it’s important to consider his comments on Cain and Abel, who provide a paradigm for his understanding of the Two Cities. This paradigm is echoed in 1 John 3:11-24.
Elsewhere, Augustine speaks of the coming termination of the City of Man; it is not eternal but temporal. It is “doomed to undergo eternal punishment with the Devil… when it is condemned to the final punishment it will no longer be a city.” [15.1,4]. Above, he locates the City of God as, properly-speaking, a heavenly city. This city “is up above.” The incredible thing is that it produces citizens that, for a time, live “here below.” These citizens are pilgrims until the resurrection.
Therefore, the City of Man is, properly-speaking, a City of the present, while the City of God properly belongs to the future. The members of the present city are those that love themselves and consider the things of the present to be ultimate. The Christian looks beyond this life to his or her greatest love, God Himself.