John Calvin on Genesis 5:4

4. And the days of Adam after he had begotten Seth . In the number of years here recorded we must especially consider the long period which 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, in order to renew the memory of the creation, the fall, and the punishment of man; to testify of the hope of salvation which remained after chastisement, and to recite the judgments of God, by which all might be instructed. After his death his sons might indeed deliver, 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 been himself the eyewitness of all these things. Yet so wonderful, 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.