This has been answered by a notable authority as follows: "Gen. 6:1-4 forms the introduction to the story of the Flood. All races have preserved the tradition of a flood; whether it was universal or local is a moot point The Jewish Scriptures leave the investigation of natural phenomena to human research. The Bible is not a scientific treatise. Its sole concern is religious and moral. Its aim is to justify the ways of God to man, and to show that natural phenomena, being controlled by God, are in harmony with divine justice. Hence, before relating the story of the Flood, Holy Writ sets forth the universal corruption which justified the destruction of the human race, with the exception of one family. Chapter 6:1-7 describes the violence and immorality prevalent in the antediluvian period. Mankind had, in course of time, fallen into two divisions--the classes and the masses. The masses were the common multitude of toilers, the ordinary 'sons of men.' The classes were the 'supermen,' 'the sons of God,' 'the mighty heroes.' The latter formed the aristocracy; they were the ruling class, the children of judges and princes. Small in number, they were physically strong and mentally vigorous, and had, moreover, appropriated a large portion of the wealth of the then known world. They should have used their power and position for the benefit of their kind, and set an example in chastity, temperance, self-restraint, justice and kindliness. Instead, they gave way to unbridled lust, to indulge which they resorted to violence. "They saw that the daughters of men (i.e., the common folk) were fair, and they took (i.e., by force) whomsoever they chose." This abuse of power was punished by the destruction of the race. The Eternal said: "My spirit shall not abide in man forever." The Hebrew word may mean 'abide as a sword in a sheath'; or it may mean 'contend with man'--the higher with the lower nature--the spirit of heaven with the body formed of dust and its instincts, of the earth, earthy; or it may mean 'My spirit shall not rule in man.' The struggle is too severe. 'Since he is but flesh, his days shall be one hundred and twenty years.' On account of the moral infirmity incident to human nature, time will be given for repentance. If the opportunity is not taken, destruction will follow the respite. Nephilim literally means 'the fallen.' On the principle of Incus a non lucendo, the term refers to the men of gigantic stature who existed in ancient times. They were the mighty men who yielded to licentious passions. The children of these illegitimate unions were also, for some generations, Nephilim of gigantic stature, famed for their physical and mental development, but morally degenerate. They were the renowned heroes of old--the mighty warriors, like the berserkers of the northern sagas."
Another view is that "the sons of God" were the Semites, who had maintained in some measure the filial relationship to God, and who now intermarried with the Cainites, who had been spiritually disowned on account of their godlessness and unbelief. All the evidence leads to the conclusion that the whole arraignment of wickedness upon the earth related to beings of flesh and blood (see Gen. 6:3) and not to supernatural beings, who, we are elsewhere told distinctly, have no distinction of sex and never marry (see Luke 20:35,36). In this view, which seems to be the correct one, the appellation "sons of God" refers to men's moral and in no sense to their physical state. There are many passages elsewhere that bear out this belief. (See Acts 17:28; Ex. 4:22,23; Deut. 14:1; Hosea 11:1, etc.)
