And Cain knew his wife; and she conceived, and bore Enoch: and he built a city, and called the name of the city after the name of his son, Enoch.
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Augustine of Hippo
Now, it is recorded of Cain that he built a city, while Abel, as though he were merely a pilgrim on earth, built none. For the true city of the saints is in heaven, though here on earth it produces citizens in whom it wanders as on a pilgrimage through time looking for the kingdom of eternity. When that day comes, it will gather together all those who, rising in their bodies, shall have that kingdom given to them in which, along with their Prince, the King of Eternity, they shall reign forever and ever.
Consider now the text: “And Cain knew his wife, and she conceived, and brought forth Enoch; and he built a city and called the name thereof by the name of his son Enoch.” It does not at all follow from these words that we must believe Cain’s first son was Enoch, as though “Cain knew his wife” must refer to their first intercourse. You have the same expression used of the first father, Adam, but not only in reference to the conception of Cain, who seems to have been his firstborn, since a little later Scripture records, “Adam knew his wife, and she conceived and brought forth a son and called his name Seth.”
His wife. She was a daughter of Adam, and Cain's own sister; God dispensing with such marriages in the beginning of the world, as mankind could not otherwise be propagated.
He built a city, viz. In process of time, when his race was multiplied, so as to be numerous enough to people it. For in the many hundred years he lived, his race might be multiplied even to millions. (Challoner)
The Hanuchta, which Ptolemy places in Susiana, (Calmet) may perhaps have been built after the flood, in the same place. Josephus says, Cain was the first who fortified a city; designing it for a retreat, where he might keep the fruits of his robberies, Antiquities 1. 3. Peirere founds his ill-concerted system of Preadamites, or of men existing before Adam, on the history of Cain exercising husbandry, building a city; as if there were any difficulty in supposing, that the arts would have made some progress in the lapse of above a century. (Haydock)