I. Man's rebellion never goes unnoticed.
First, in verses 1-4 we see that man's rebellion never goes unnoticed. When God brings judgment in Scripture, He always gives the reasons for that judgment, and the judgment is always proportionate to the sin which is being punished. In Genesis 6:1-4, God is beginning to tell us why the world prior to Noah deserved to be judged so severely. The very scale of God's judgment in the flood is indicative of the heart depravity of the people. It is a picture of the perversity of their hearts, the pervasiveness of their sin, and the heinousness of their sin in the sight of God.
Verses 1 and 4 present a bit of a conundrum. Orthodox interpreters have had a hard time explaining exactly what they mean. What does “sons of God” mean? What are the Nephilim? There are at least three major interpretations, but the strongest we can offer is that Moses is speaking here of intermarriage between the line of Seth, the godly line of salvation, and the line of Cain, the line of destruction that had rejected the Lord.
The theological and contextual reasons for this interpretation are strong. First, the concept of a godly line has already been established in Genesis 3-5. Second, the concept of sonship based on divine election is a very important idea not only in Genesis but in the whole of the Old Testament. Thirdly, when you look at the first five books of the Bible, there are numerous warnings given by Moses against intermarriage of believers and unbelievers. Finally, if you look at verse 2, you’ll see an interesting phrase at the very end of that verse: "Whomever they chose." Moses seems to be indicating here that the sons of God, those in the line of Seth, in rebellion against His will chose anybody that they wanted. The point is that they allowed superficial reasons to bring about intermarriage between those who believed in the Lord and those who did not believe.
Verse 3 shows you God's initial response to this situation. He declares that He will not strive with man forever. But, though sin is rampant, human accomplishments and power are still apparent in this age. You see this in verse 4 where we are told about Nephilim. Yet, though man may seem to be master of his domain, when he rebels against the Lord there will be consequences. The women were beautiful, the men were mighty and did feats of renown, but God was unimpressed. And so judgment came. Everything may seem to be going well in our own lives on the surface, but if we are out of accord with God judgment will follow. By the way, this passage is a very pointed reminder that those who are contemplating marriage must marry in the Lord. With divine authority it reminds us how important it is in our dating life to begin with purity with regard to faith lest we enter into a relationship that will then bring displeasure to the Lord and disaster to ourselves.
II. Sin brings judgment.
We see in verses 5-7 an acute description of man's depravity and God's judgment. Again we learn that we are the problem and that sin brings judgment. The Lord, we are told, saw man's heart and its outworking. Notice the contrast between this and the creation account. Previously the Lord saw, and it was good. But now we learn that the Lord saw, and every imagination of his heart was only evil continually as the Lord looks upon man.
Man's depravity is described in verses 5 and following both externally and internally. The latter, his heart wickedness, is said to be complete and continuous. Look at the almost poetic phrase. “Every intent of the thoughts of his heart was only evil continually.” Paul, too, tells us that there is no good thing in the human heart which is not regenerated. This is not just a picture of a broken-down society. It's a picture of the potential in all our hearts. God sees our hearts, so there is no escaping an accounting for what we think and for our actions.
Now, something very interesting and very hard is said in verse 6. We are told that the Lord was sorry that he had made man on earth. This an anthropomorphism, or, more specifically, an anthropopathism. An anthropopathism is an ascription of human emotion to God. In this passage it is used in order to emphasize the strength of God's disgust for man's sin. It does not mean that God made a mistake that He wished He hadn't made. Elsewhere in the Old Testament, the writers of the Old Testament are emphatic that God does not change His mind or make mistakes. For example, in 1 Samuel 15:35 it seems like God is saying something similar about Saul, that He regretted making him king, but verse 29 has already said that God does not change His mind like men. So this language is intended to make emphatic how God responds to sin. The language is not to be played down. The language ought to be played up to emphasize how much God hates sin.
The Lord's judgment then comes following on Genesis 6:6. He promises to blot out all the living creatures on earth. It is very clear that man is the problem here, and until we own that we are the problem, we are not ready for the grace of the Lord. It is also clear here that sin is not circumstantial; it's sin of the heart. So we must not tone down the strong language of Genesis 6:6-7 because that language is descriptive of the Lord's reaction to sin. It ought to make us fear. Genesis 6 - 9 is a picture of what sin deserves, and it's a foretaste of a universal judgment that will come.
III. God's grace is man's only hope.
There’s one last brief thing we see in verse 8: the grace of God towards one man becomes the salvation of humanity. Here we see God's grace is the only hope. Genesis 6:8 is the first occurrence of the word grace in the Bible: “But Noah found favor (grace) in the eyes of the Lord.” What does grace mean? Unmerited, divine favor in spite of positive demerit. In other words, though all deserve punishment for sin, grace shows favor in spite of the sin. It's interesting that that's the very last phrase of what Moses calls the book of Adam. The very next sentence in this book will be the book of Noah. So we have here in verses 7-8 the only two possible responses of the Lord to sin, either complete judgment or complete salvation.
Source : http://www.onlinemadison.com/Content/EDITORIALS/Editorials/Article/DUNCAN-A-World-of-Corruption-and-God-s-Singular-Grace/3/3/40925