John Calvin Commentary Genesis 5:4

"and the days of Adam after he begat Seth were eight hundred years: and he begat sons and daughters." — Genesis 5:4 (ASV)

And the days of Adam after he had begotten Seth. In the number of years recorded here, we must especially consider the long period that the patriarchs lived together. For through six successive ages, when the family of Seth had grown into a great people, the voice of Adam might daily resound, to renew the memory of the creation, the fall, and the punishment of man; to testify to the hope of salvation that remained after chastisement; and to recite the judgments of God, by which all might be instructed. After his death, his sons might indeed pass on, as from hand to hand, what they had learned to their descendants; but far more efficacious would be the instruction from the mouth of him who had himself been the eyewitness of all these things. Yet so astonishing, and even monstrous, was the general obstinacy, that not even the sounder part of the human race could be retained in the obedience and the fear of God.