1When men began to multiply on the surface of the ground, and daughters were born to them, 2God’s sons saw that men’s daughters were beautiful, and they took any that they wanted for themselves as wives. 3Yahweh said, “My Spirit will not strive with man forever, because he also is flesh; so his days will be one hundred twenty years.” 4The Nephilim were in the earth in those days, and also after that, when God’s sons came in to men’s daughters and had children with them. Those were the mighty men who were of old, men of renown.
5Yahweh saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of man’s heart was continually only evil. 6Yahweh was sorry that he had made man on the earth, and it grieved him in his heart. 7Yahweh said, “I will destroy man whom I have created from the surface of the ground—man, along with animals, creeping things, and birds of the sky—for I am sorry that I have made them.” 8But Noah found favor in Yahweh’s eyes.
9This is the history of the generations of Noah: Noah was a righteous man, blameless among the people of his time. Noah walked with God. 10Noah became the father of three sons: Shem, Ham, and Japheth. 11The earth was corrupt before God, and the earth was filled with violence. 12God saw the earth, and saw that it was corrupt, for all flesh had corrupted their way on the earth.
13God said to Noah, “I will bring an end to all flesh, for the earth is filled with violence through them. Behold, I will destroy them and the earth. 14Make a ship of gopher wood. You shall make rooms in the ship, and shall seal it inside and outside with pitch. 15This is how you shall make it. The length of the ship shall be three hundred cubits, its width fifty cubits, and its height thirty cubits. 16You shall make a roof in the ship, and you shall finish it to a cubit upward. You shall set the door of the ship in its side. You shall make it with lower, second, and third levels. 17I, even I, will bring the flood of waters on this earth, to destroy all flesh having the breath of life from under the sky. Everything that is in the earth will die. 18But I will establish my covenant with you. You shall come into the ship, you, your sons, your wife, and your sons’ wives with you. 19Of every living thing of all flesh, you shall bring two of every sort into the ship, to keep them alive with you. They shall be male and female. 20Of the birds after their kind, of the livestock after their kind, of every creeping thing of the ground after its kind, two of every sort will come to you, to keep them alive. 21Take with you some of all food that is eaten, and gather it to yourself; and it will be for food for you, and for them.” 22Thus Noah did. He did all that God commanded him.
"The oldest, and likely the most widely held, interpretation is that the “sons of God” are fallen angels (demons). This was the interpretation most favored in ancient Judaism and the early church (cf. 1 Pet. 3:19, 20; 2 Pet. 2:4; Jude 6). The phrase “sons of God” is clearly used elsewhere of angelic hosts in God’s heavenly court (cf. Job 1:6; 2:1; 38:7). Moreover, the narrator seems to contrast “man” and “the daughters of man” with the “sons of God” in Genesis 6:1, 2."
"This position is not without difficulties, however, the most substantial of which is the idea of fallen angels having physical relations with women. Scripture gives instances of angels engaging in human activities such as eating (Gen. 18:1, 2, 8; 19:1, 5), but surely sexual intercourse is a step beyond! Jesus makes a similar point in Matthew 22:30: “For in the resurrection they neither marry nor are given in marriage, but are like angels in heaven.” William F. Cook, Who Are the Sons of God in Genesis 6? Some of these church fathers were Justin Martyr, Eusebius, Clement of Alexandria, Origen, and Commodianus. Douglas et al. 2011, p. 1384
Most early church fathers in the first three centuries believed that the sons of God were angels who had unnatural sex with women and bore children. This perspective changed with St. Augustine of Hippo, who argued in the City of God that the sons of God were descendants of Seth.
The argument that Jesus taught that angels in heaven did not marry has no impact on the Genesis 6 passage since the angels in Gen 6, as accepted by Jude and Peter, were not unfallen angels in heaven but evil angels who were part of the fallen angels, or unfallen angels involved in human activity before the flood, who also fell. The ability to have sex and bear children appears to be a major argument against the "angel" view, but one must remember that angels in Gen 18 came in human form, along with Yahweh, ate food and drank, and had their feet washed. Moreover, in the New Testament, angels always appear in a male human form." See my article, Will We Eat and Drink in the Coming Kingdom of God? An Interpretation with Origen, Cerinthus, the Church Fathers, and the Literal Interpretation of the Scriptures on the Nature of the Millennial Kingdom.
6. And the angels who kept not their first estate but left their own habitation, he hath reserved in everlasting chains under darkness the judgment of the great day.
The strongest text that associates the sons of God in Genesis 6:1-4 is the Book of Enoch (6:11), referenced by the writer Jude in 1:6: "When the son of men had multiplied, in those days, beautiful and comely daughters were born to them. 2/ And the watchers, the sons of heaven, saw them and desired them. And they said to one another, "Come, let us choose for ourselves wives from the daughters of men, and let us beget children for ourselves."[1]
The ancients had many traditions of deities cast out from heaven. See notes on Genesis xi. 8. Jove, deceived by the goddess Ate, is represented by Homer as having cast her forth, forbidding her return to Olympus with an oath.
"From his ambrosial head, where perch'd she sat,
He snatched the fiery goddess of debate,
The dread, the irrevocable oath he swore,
The immortal seats should ne'er behold her more;
And whirl'd her headlong down, for ever driven
From bright Olympus and the starry heaven." Il 1. xix. v. 126
Jove declares that any deity who interferes in the strife between the Greeks and
Trojans—
"Far, oh far, from steep Olympus thrown,
Low in the dark Tartarean gulph shall groan,
With burning chains fixed to the brazen floors,
And lock'd by Hell's inexorable doors.''-Hom. ll, 1. xix v. v.3.
" With enduring chains
He bound Prometheus, train'd in shifting wiles,
With galling shackles fixing him aloft."-Hes. Theog. v. 521.
[1] George W. E. Nickelsburg and James C. VanderKam, 1 Enoch: A New Translation (Minneapolis: Fortress Press), p. 23.