Augustine now proceeds to trace the history of the two cities through an examination of the early story of humanity as outlined in Genesis 4-9: “I classify the human race into two branches: the one consists of those who live by human standards, the other of those who live according to God’s will. I also call these two classes the two cities, speaking allegorically. By two cities I mean two societies of human beings” (595). He notes that both of these cities are evident in Adam and Eve’s own family: One son, Cain, follows his violent passions and represents the earthly city, and the other son, Abel, follows God’s way. Cain’s murder of Abel strikes Augustine as a foundational event in the history of the earthly city, paralleling Romulus’s fratricide of Remus in the story of Rome’s founding.

Augustine argues for the historical accuracy of the biblical narrative, answering questions of interpretation regarding the long lifespans recorded there, the accounts of “giants,” and other peculiar features in the early chapters of Genesis. Near the end of Book 15, he deals with the account of Noah’s flood and the question of whether it suggests that God has emotional passions and changes of mind like humans.