The City of God

One of Augustine's most famous works, this book tells of the fall of the Roman Empire and the rise of the holy and righteous City of God from the ashes. While building a utopia much like The Republic does, Augustine uses sound theology to build the foundations of the cities morals and lawful authority. Many theologies, especially within the Catholic tradition, owe their beginnings to the teachings of Augustine and this work is one of the starts of the field.