After humanity had fallen deep into sin, the gate was open for a torrent of evil to come forth from the human generations. The Lord said "My Spirit shall not always strive with man forever, for he is flesh, therefore shall his days be a hundred and twenty years" (Gen. 6:3). Human longevity will be cut down from five or six hundred years, or more, to an average of about a hundred and twenty. This will put the brakes on the sinfulness of the human race for a period. It again with be cut down to seventy years in order to slow down the evil that comes forth from the human species.
The Rabbis write:
"Despite the fact that man is created in the Divine image, he has proved by his proneness to err that he is 'flesh'; i.e. the earthly side of his nature too readily overpowers the spiritual." God gave some time for mankind to repent and change his ways.
The Spirit of God is here active. God claims the uniqueness of the Spirit. He calls Him "My Spirit." A "force" cannot strive. Only an intelligent being can act and strive as the Spirit does in this context.
Though the Bible does not speak of the Holy Spirit indwelling Adam before the fall, Abraham Kuyper makes an interesting statement:
"We arrive at the conclusion, that in Adam all goodness was wrought by the Holy Spirit, according to the holy ordinance which assigns to the Third Person in the Trinity the inward operation of all rational beings. … Adam's nature was created and calculated to be a vehicle of the Holy Spirit's operations." "By the fall, man had forfeited all hope of further efforts of God's grace," Leupold notes.
—Dr. Mal Couch (3/11)