Even the structure of Genesis puts the emphasis on the heavenly city, rather than the earthly one-- the latter "begins and ends with a murderer," the former "begins with the man who hoped to call upon the name of the Lord God, for the invocation of God is the whole and highest preoccupation of the City of God during its pilgrimage in this world."
Again, both of these cities are under the sovereign rule of the Lord:
God fashions two kinds of pottery: the vessels fashioned by His wrath and fit only for contempt and the vessels made by His mercy and meant to be honored. To the former He pays in punishment the doom they earn; to the latter He bestows, as a gift of grace, a destiny they never could have deserved.What's more, this division between the two cities is intentional on God's part:
God's purpose in this [division] is that the heavenly City, during its exile on earth, by contrasting itself with the vessels of wrath, should learn not to expect too much from the freedom of the power of choice, but should trust in the 'hope to call upon the name of the Lord God.Chapter 22:
The two cities, however, are mingled together in this world. We can discriminate between them by examining their different responses to beauty. The earthly city desires physical beauty. And while physical beauty is no bad thing--indeed, it is a gift from God-- it is not to be considered the most important kind of beauty. The most important kind of beauty, the love of which sets the heavenly city apart from the earthly one, is the love of virtue.