1When man began to multiply on the face of the land and daughters were born to them, 2the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. 3Then the LORD said, "My Spirit shall not abide in[a] man forever, for he is flesh: his days shall be 120 years." 4The Nephilim[b] were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown.Someone once mentioned to me that maybe the "Nephilim" were fallen angels, or even possibly aliens from out of space. Genesis 6 certainly gives us a lot of questions. Who are these "sons of God," who are the Nephilim which seems to be a plural derived from naphal which means "to fall"?
Well, just a few days ago I read Geerhardus Vos' treatment of this passage and he makes some amazing points that settle it clearly for me.
1) This period is marked by an increase in wickedness and evil upon the land while the good does not seem to do anything more than retreat. The wickedness of man was great and the intentions of his heart was only evil continually. (let's set aside the "and the LORD was sorry"+God is immutable question).
5The LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. 6And the LORD was sorry that he had made man on the earth, and it grieved him to his heart. 7So the LORD said, "I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them." 8But Noah found favor in the eyes of the LORD.2) There are two lines of people here, one of them is the Cainites, decended from Cain and into Lamech who begins polygamy and declares his independence from God. The other is the line of Seth, which emphasizes itself in Enoch, who walked with God and was no more.
3) Thus we understand verse 2, "the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose" to mean that the people decended from the line of Seth (sons of God) are falling away and intermarrying with the line of Cain. They are the sons of God because they were part of God's promise and people, the "daughters of man" mean that they are men and nothing more, in contrast to those who were not only men, but also sons of God.
This nicely explains the increase of sin and wickedness, the falling away of God's people, and whole "Nephilim" issue rather neatly.
I thought that was pretty cool.