Augustine says that the City of Men is concerned with generation while the City of God deals also with regeneration. He has been looking at the first several generations of humanity as outlines in Genesis – how they procreated, how many sons and daughters are left out of the genealogies, whether brothers and sisters might have married one another, why that practice might have ended, etc. These are questions of generation, related to both cities and relevant to all humanity. Everyone ought to be concerned with how more human beings come into being because the customs of generation speak to and reflect what we believe humanity to be. Our current historical moment has shown that humanity wants to redefine itself, and this redefinition is demonstrated most clearly in the realm of sex. Humanity is an object or a consumer – sex is what I want it to be and you can’t tell me to limit my options, though I am free to limit them myself temporarily, if I so choose. And babies are objects that I freely choose, the limits of sex (whether I’m having sex with a partner who can get pregnant or not, or whether there is even a male to fertilize an egg or a female to provide one) or even pregnancy (if I am pregnant, I can simply abort) do not limit my free choices. Human beings as consumers freely make choices. As the Supreme Court has told us recently, people have the right to define and limit themselves and their own happiness. This, of course, is utter gibberish, but this is the redefinition of humanity that we are experimenting with. The human person is not primarily or foundationally a consumer, as Augustine would know.
Instead, we are created, loved, relational beings who sin and need regeneration. The City of Man is about generation, and is currently trying to redefine humanity. God has founded His City as a place of regeneration, to deal with the effects of the Fall and its consequent fallen generation. God is making us new in this City. He defines us, even defining our happiness and purpose.
May He bless you with regeneration in His City!