A very interesting passage of scripture occurs in Genesis, chapter the sixth:
1 When men began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of men were beautiful, and they married any of them they chose. 3 Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years." 4The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown. (Genesis 6:1-4)
Who are these Nephalim (or, as some translations use, giants?)
Well, there is the sexy explanation, which is almost certainly incorrect. And that is: the Nephilim are the result of interspecies sexual congress between angels (sons of God) and human women (daughters of men). Among angels, as with humans, it would appear that the males of the species are licentious pigs. This explanation even makes a connection with the New Testament where we read, in Paul's rather enigmatic explanation of why women should wear head coverings in church:
For this reason, and because of the angels, the woman ought to have a sign of authority on her head. (1 Cor. 11:10)
The idea being, according to this reasoning, that angels are in attendance, and to help them keep their mind on worship instead of wandering into the gutter women should hide their beauty.
I can only say that I agree with John Calvin who wrote, concerning this explanation:
That ancient figment, concerning the intercourse of angels with women, is abundantly refuted by its own absurdity; and it is surprising that learned men should formerly have been fascinated by ravings so gross and prodigious. (From his commentary on Genesis.)
What is the more plausible and less fanciful explanation? Well, for starters you need to read Genesis chapters 4 and 5 and see the genealogies of Cain and Seth, his brother who was born after the murder of Abel.
Seth's family tree includes the righteous Enoch who walked with God and apparently did not suffer physical death but instead had a sort of personal rapture. Enoch was the father of Methuselah, the longest lived man, who made it to age 969. And Methuselah 's grandson was a "righteous" man by the name of Noah.
Cain's descendents are, by comparison, a rogues gallery. They include Lamech who is credited with introducing the world to polygamy and who wrote poetry that boasted of his murderous exploits. (Genesis 4:19-24).
Add to this the realization that "son" in the bible does not always refer to a biological offspring, but rather one who is worthy to be a descendent, regardless of bloodline. For example the Pharisees were warned not to count on the fact that they were blood sons of Abraham. God, they were told by John the Baptist, could raise sons of Abraham from the stones. And Jesus likewise told them that, in spite of the blood that flowed in their veins, they were not the sons of Abraham but the sons of the devil.
So here we find the probable explanation: The sons of God were the "good" bloodline that descended from Seth. The "daughters of men" were exotic beauties in the "bad" bloodline of Cain. The progeny of the ill advised union between the two groups was worse yet, including a group of giants who it appears was something like a gang of thugs. (Brutes is perhaps a better word than giants.) Calvin blames Jerome for a faulty translation, pointing out (among other things) the Moses never actually comments on their size. And Matthew Henry writes, in his commentary on Genesis:
[We] read of a nation that was multiplied, not to the increase of their joy, (Isa. 9:3.) Numerous families need to be well-governed, lest they become wicked families. Mixed marriages (v. 2): The sons of God (that is, the professors of religion, who were called by the name of the Lord, and called upon that name), married the daughters of men, that is, those that were profane, and strangers to God and godliness. The posterity of Seth did not keep by themselves, as they ought to have done, both for the preservation of their own purity and in detestation of the apostasy. They intermingled themselves with the excommunicated race of Cain: They took them wives of all that they chose. But what was amiss in these marriages? 1.) They chose only by the eye: They saw that they were fair, which was all they looked at. 2.) They followed the choice which their own corrupt affections made: they took all that they chose, without advice and consideration. But, 3.) That which proved of such bad consequence to them was that they married strange wives, were unequally yoked with unbelievers, (2 Cor. 6:14).
What we have, in effect, is the first of many warnings in the bible against syncretism. So important is this to God that later he will later instruct Joshua to commit genocide and engage in ethnic cleansing (showing no concern that 21st century atheists will use those commands to label God a sadist and a mass murderer) to eliminate sources of religious pollution from the peoples of Canaan.
We have, in the proper way of looking these events leading up to the flood, a very familiar theme: the world is divided into two groups, call them Seth's line and Cain's line, (Old Testament) Jews and Gentiles, elect and unelect, vessels of mercy and vessels of destruction, believers and unbelievers. Whatever you call them, God will justly give the one group the punishment due to all men. To the other group he will demonstrate his mercy and glory by sparing them by grace and by the redemptive power of Christ's blood.