There was going to be a withdrawing of the Holy Spirit. Now, whether that was to take place when the flood came about, or whether it would be even before the flood is unclear, but the Lord says, ‘My spirit shall not always strive with man [in grace], for that he is also is flesh’, which seems to imply that the sons of God, the descendants of Seth have become fleshly, like those of Cain.
We learn that there is a work of the Spirit of God in every soul of man, even in those who are not converted. God’s Spirit strives for a limited time to awaken the conscience of those who have reached the age of accountability, to cause them to acknowledge their sin and come for forgiveness. He does not work irresistibly as in the elect, bringing them most definitely to bow the knee to Christ, but he works sufficiently to leave them without excuse. To resist this work of the Spirit is a most dangerous thing to do, because once the Spirit is withdrawn, there is no possibility of salvation, and his patience with the obdurate lasts only a limited time. The right response is to pay close attention to the voice of conscience and to pray for more light and for a willing spirit with which to hear what God’s Spirit says.
We can identify with Noah. I can remember as an unconverted youngster frequently seeing in central London streets the sandwich boards, which have comparatively gone out of fashion, the bold witnesses who would walk up and down the streets all day with their boards saying that judgment was nigh. As a youngster, you looked at this and thought, he must be mad. What a crazy thing, even at a time when there was general concern and fear about atomic bombs and hydrogen bombs, such as we don't see today. Those boards were so ridiculous. You can imagine the same scorn for Noah for 120 years.