Book 14 promises to be a look at the two cities, the one lived “according to the spirit” and the other “according to the flesh”. Augustine begins by defining his terms: “according to the flesh” does not mean living according to a philosophy that values the body versus philosophies that value the min for spirit. Instead, life “according to the flesh” means all human life, where humanity lives like the humanity under Adam. Augustine follows Scripture in claiming that all humanity lives life according to the flesh and deserves death and the second death, but that God has interjected Himself into humanity – “the Word became flesh” – and has saved many by His grace.
Revelation uses different language to mean the same thing; rather than talking about life “according to the flesh” and “according to the spirit”, Revelation talks about “those who dwell on the earth” and “those who dwell in heaven”. The concepts are distinct but similar and the two groups inhabit cities – City of God/New Jerusalem vs. City of Man/Babylon. There are some significant differences in the language around the cities and their inhabitants, though. Life according to the flesh talks about the source of life for a person, from within themselves; living as a dweller of earth talks about a location and a setting. And the City of God signals a city and its owner; the New Jerusalem points to the historical and storied city of Jerusalem, in which the people of God lived and where they built a temple in which God might live. Perhaps the “City of God” language is helpful for dealing with a pagan audience. I would suggest, though, that for Christians “New Jerusalem” might be more suggestive. I may be obnoxious in my blogging and talk about the New Jerusalem where Augustine talks about the City of God. We’ll see.