One of the best books ever written. Augustine wrote this just as Rome was coming to an end. Part of the impetus was to show that the City of God was not confined to the Roman Empire, but would outlast any earthly empire. The amount of detail he poured into describing the pagan culture of his time was also amazing. Also, he offers some fascinating theological insights towards the end of the book.

If you want to understand Western Christendom, you really have to read this book from cover to cover.