“Thus there are two lines of descent, one from Seth and the other from Cain; and their separate lists convey the notion of the two cities, the Heavenly City on pilgrimage in this world, and the earthly city, which longs for earthly joys, or clings to them, as though they were the only joys.” [The City of God, 15.15]
The genealogies listed in Genesis provide further evidence that God wants us to read the Bible as a tale of Two Cities. Spiritually-speaking, one descends from Seth, the City of God on pilgrimage. The other is the ancestry of Cain, the City of Man.
The clarification Augustine makes in the above quote is a helpful one. Describing Cain’s city, he speaks of their longing “for earthly joys… as though they were the only joys.” This is significant since the clear suggestion is that the opposite is true of Christians – they don’t see earthly joys as the only joys. In other words, a Christian may find joy in this life – good things that God created and Scripture affirms – but there is a far greater joy that they anticipate, reigning with Christ in His heavenly kingdom.