Gen 6:1-5"And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them.
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
There were giants in the earth in those days, and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men where were of old, men of reknown.
And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was evil continually."
(There are no real answers to the mystery of the Nephilim. Who or what they were. But, there seems to be quite a difference of opinion. I have quoted some of the comments from modern commentators and some comments from the writers of antiquity.
With all the information available, [which isn't alot] there is nothing concrete.
Some feel the 'sons of God' refers to the godly line of Seth, and some feel this describes 'fallen angels'.
Some even feel this is where mythology originated. That the ancient legends of Mythology had their beginnings at this time in history. And that the myths grew out of actual happenings and is not mere inventions of the mind.Perhaps the ancient civilizatiions of yesterday did not just dream up their gods after all.
The reader will have to be the judge of what value, [if any] is to be placed on the information we have.
Some are of the opinion that these 'giants' may have been utilized by Egypt in the construction of the great pyramids.
If these Nephilim were associated with Egypt this might account for how those high stones were brought together.)
(The writers of antiquity seem to have the opinion that these 'sons of God' were angels. Today, many scholars feel this is not the case.)
Bible Reader's Companion p. 29 Insight by Lawrence Richards
"Sons of God. This obscure passage has been taken as intermarriage between the 'godly line of Seth' and the line of Cain. But Jewish commentators saw it as forbidden intercourse between fallen angels and human women, which produced a race of Nephilim (giants)."
(Hayford Bible Handbook says almost the same as the quote above.)
Unger's Bible Handbook p. 48 The Flood
"Two common views are held as to the cause of the flood: (1) the intermarriage of the godly Sethites with the ungodly Cainites: (2) the intermarriage of 'the daughters of men'. i.e. 'women in the flesh'. with the bene Elohim, 'sons of God', i.e. angels.
The main argument against the first view is that it fails to meet the scope of the passage or account for the nephilim (Septuagint [gigantes]; not 'giants' but of mixed human and angelic birth, like the Titans of Greek and Roman mythology, who were partly human and partly divine - angelic). The argument that angels are sexless because marriage is unknown among them (Mt 22:30) ignores the fact of evil fallen angels as well as the pure unfallen spirits (cf. Gen 19:1,5). Moreover, bene Elohim is an expression used consistently of angels in the O.T. (Job 1:6: 2:1 etc...); and the breaking down of God - ordained orders of beings (c.f. II Pet 2:4-5: Jude 6) is the only express of this passage that will satisfy its scope."
(Now, some comments from antiquity)
The Sibylline Oracles- believed to be written between 160 B.C and before A.D. 150
The early writers know of one Sibyl. Later more were added: Clement of Alexandria mentions nine, Varro lists ten.
The Sybil says in Book 1. 120-125
"The fifth generation
Again he made afterward another far inferior race of men, for whom thereafter immortal God fashioned no good, since they suffered many evils. For they were insolent, much more than those Giants. crooked ones, abominably pouring forth slander."
I Enoch seems to have been written at the latest by the end of the first century A.D.
Enoch says in Book 1; 6:1-2
"In those days, when the children of man had multiplied, it happened that there were born unto them handsome and beautiful daughters. And the angels, the children of heaven, saw them and desired them; and they said to one another, 'Come, let us chose wives for ourselves from among the daughters of man and beget us children."
I Enoch Book 1; 7:1-6
"And they took wives unto themselves and everyone (respectively) chose one woman for himself, and they began to go unto them. And they taught them magical medicine, incantations, the cutting of roots, and taught them (about) plants. And the women became pregnant and gave birth to great giants. Whose heights were three hundred cubits. These (giants) consumed the produce of all the people until the people detested feeding them. So the giants turned against (the people) in order to eat them. And they began to sin against birds, wild beasts, reptiles, and fish. And their flesh was devoured the one by the other, and they drank blood. And then the earth brought an accusation against the oppressors."
Book of Jubilees - Date 125-75 B.C.
the author says:"And he wrote everything, and bore witness to the Watchers, the ones who sinned with the daughters of men because they began to mingle themselves with the daughters of men so that they might be polluted. And Enoch bore witness against all of them."
Jubilees 5:1-11"And when the children of men began to multiply on the surface of the earth and daughters were born to them, that the angels of the Lord saw in a certain year of that jubilee that they were good to look at. And they took wives for themselves from all those whom they chose. And they bore children for them; and they were the giants. And injustice increased upon the earth, and all flesh corrupted its way; man and cattle and beasts and birds and everything which walks on the earth. And they all corrupted their way and their ordinances and they began to eat one another. And injustice grew upon the earth and every imagination of the thoughts of all mankind was thus continually evil.
And the Lord saw the earth, and behold it was corrupted and all flesh had corrupted its order and all who were on the earth had done every sort of evil in his sight. And he said, 'I will wipe out man and all flesh which I have created from upon the surface of the earth.' But Noah alone found favor in the sight of the Lord.
And against his angels whom he had sent to the earth he was very angry. He commanded that they be uprooted from all their dominion. And he told us to bind them in the depths of the earth, and behold, they are bound in the midst of them, and they are isolated. And against their children a word went forth from before his presence so that he might smite them with the sword and remove them from under heaven. And he said, 'My Spirit will not dwell upon man forever; for they are flesh, and their days will be one hundred and ten years. And he sent his sword among them so that each one might kill his fellow and they began to kill one another until they all fell on the sword and they were wiped out from the earth. And their parents also watched. And subsequently they were bound in the depths of the earth forever, until the day of great judgment in order for judgment to be executed upon all of those who corrupted their ways and their deeds before the Lord. And he wiped out every one from their places and not one of them remained whom he did not judge according to all his wickedness."