City of God

But that which God said, “Their days shall be a hundred and twenty years,” is not to be understood as a prediction that henceforth men should not live longer than 120 years—for even after the deluge we find that they lived more than 500 years—but we are to understand that God said this when Noah had nearly completed his fifth century, that is, had lived 480 years, which Scripture, as it frequently uses the name of the whole of the largest part, calls 500 years. Now the deluge came in the 600th year of Noah's life, the second month; and thus 120 years were predicted as being the remaining span of those who were doomed, which years being spent, they should be destroyed by the deluge.

And it is not unreasonably believed that the deluge came as it did, because already there were not found upon earth any who were not worthy of sharing a death so manifestly judicial,— not that a good man, who must die some time, would be a jot the worse of such a death after it was past. Nevertheless there died in the deluge none of those mentioned in the sacred Scripture as descended from Seth. But here is the divine account of the cause of the deluge: “The Lord God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.

And it repented the Lord that He had made man on the earth, and it grieved Him at His heart. And the Lord said, I will destroy man, whom I have created, from the face of the earth; both man and beast, and the creeping thing, and the fowls of the air: for I am angry that I have made them.”