This book will set out the history of the two cities from the beginning to their end (which will be discussed at the end of City of God).
Both cities are paralleled in Cain and Abel--the one of the city of man and the other of the City of God. We all start out in the city of man and the stock of Cain, but can be bought through regeneration in Christ into the City of God and the family of Abel:
The fact is that every individual springs from a condemned stock and, because of Adam, must be first cankered and carnal [a better translation here is 'wicked and worldly'], only later to become sound and spiritual by the process of rebirth in Christ.This transition from the first city to the second is one that comes directly from God, as He by grace transforms us from wicked vessels into holy ones, who have no home in this world but (as Abel) long for the heavenly City of God.
Yet, it is not as if the heavenly City has no place on earth-- "A shadow, as it were, of this eternal City has been cast on earth, a prophetic representation of something to come rather than a real presentation in time. Yet this shadow" despite being just a symbol is still real. We see this over and over in Scripture as the authors write about how this heavenly City defines us and shapes our actions now. We see the community of the City of God being gathered even in this world not necessarily in visible form, but active enough that its "presence serves as a shadow of the heavenly City." The former city is shaped by nature, "flawed by sin" and includes the whole world. The latter City is shape and built only by grace.