Who were the “sons of God” that fathered the Nephilim? Some have suggested that they were worshipers of God, as opposed to the other wicked men. This could hardly be the case if we follow the context. The account says that their marriage to “the daughters of man” caused the wickedness to increase substantially. Noah and his family of a wife, three sons, and their wives were the only ones walking with God at that time.―Genesis 6:9; 8:15, 16; 1Pe 3:20.
Therefore, if we were to suggest that these “sons of God” were merely men, this would beg the question, why would their offspring be referred to as “the mighty men who were of old, the men of renown,” more so than the other wicked men, or especially Noah and his family? Moreover, the question arise as to what would be so special, if they were just men, for the account to mention their marriage to “the daughters of man” as though that was special in some way? Men had been marrying women and having children for some 1,500 years at this point.
The understanding that these “sons of God” were disobedient angels, an interpretation that has been around since the beginning is the best choice. The same expression “sons of God” is found in Job 1:6 and Job 38:7, and is applied to angels. This interpretation is supported by the apostle Peter as well, for he writes, “he [Jesus] went and proclaimed to the spirits in prison, because they formerly did not obey, when God’s patience waited in the days of Noah, while the ark was being prepared.” (1 Pet. 3:19-20) Moreover, Jude adds weight to this position as well, when he writes, “the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day.” (Jude 6)
These rebel angels had the power at one time to material in human form, just like the ones that remain faithful to Jehovah God, as they delivered messages for Him. (Gen. 18:1, 2, 8, 20-22; 19:1-11; Josh. 5:13-15) The “proper dwelling” that Jude speaks of is heaven, to which these angels abandoned, to take on human form, and have relations that were contrary to nature with the “the daughters of man.” (Dan. 7:9-10) The Bible intimates that these rebel angels were stripped of their power to take on human form, as you never hear of it taking place again after the flood, only spirit possession after that. These disobedient angels are now “spirits in prison,” who had been thrown into “eternal chains under gloomy darkness,” which is more of a condition of limited powers, not so much a place, like a maximum-security prison.―1 Peter 3:19; 2 Peter 2:4; Jude 6.