Although Genesis 6:3 stands in the midst of a passage that is difficult to interpret (see Who Are the Sons of God?), the meaning of it in particular is much easier to understand. Regardless whether the sons of God are angels, Genesis 6:3 is clearly talking about man. It was man who was the problem; it was the wicked actions of corrupt men who brought about the flood.
Then the LORD said, "My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years." (Genesis 6:3)Some have misinterpreted Genesis 6:3 and believed that God was making it impossible for a man to reach an age of over 120 years. However, this idea can be easily refuted by the simple fact that after the flood, the first few generations of men lived well over 120 years.
The valid interpretation of Genesis 6:3 is that in 120 years the flood would devastate the earth. God would strive with man for 120 more years. Man would have 120 more years to repent. Man would have 120 more years of mercy; 120 more years of life.
This passage proclaims both a glorious and terrifying reality: there is a time when God's mercy comes to an end. There comes a time in the life of every unconverted man in which God no longer strives with him. For the men in the pre-flood world, they were given 120 more years. But for those living today, there is even less time to repent. Men do not get 120 years to repent today (and most are incapable of even living to such an age).
This passage stands in contradiction to many of the so called Christian teachings of the day. This passage does not teach that there is always another chance. This passage does not teach that God is so deeply in love with man that He would never send him to hell. Rather, this passage teaches that there comes a time when there is no longer a second chance. God's wrath is real. Just as with the pre-flood world, there comes a time for every man who denies God that God's wrath will be released and there will no longer be any hope.
Praise God for His abundant mercy! Praise God for His Son who bears the wrath that His people justly deserved!
The Total Wickedness of the Pre-Flood World - Genesis 6:5
Natural Death - Genesis 5:4-5
Righteous Men of the Pre-Flood World - Genesis 4:26
Who Are the Sons of God? (Part 1) - Genesis 6:1-4
State of the World (2948 BC): Noah's Birth - Genesis 5:28