Often thought of angels
That is a question that I regularly encounter. It is indeed an intriguing part from the Bible, that story about the sons of God who came to the daughters of men. Let's read first what the bible says.
"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." 4 The 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."
There is a lot of speculation about these texts. Often thought of angels. In itself not such a crazy assumption when you compare bible with bible because angels are called sons of God in Job. But are that all the texts which we can involve? In the OT yes, but there is an interesting text in Matthew. Jesus replies here on a question about the resurrection, the interesting part is that He is using angels as an example. Angels do not marry!
"At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven".
So Jesus explains here, in an indirect way, angels do not marry. The sons of God in Genesis 6 can therefore not be angels. Moreover, angels are spirits, and spirits can not have intercourse with people in a way they can bring forth children.
Now I once heard an interesting theory. It was stated that these sons of God are fallen angels, demons. These demons saw the daughters of men were beautiful. Those daughters are the nations that emerged on the earth. Think 'daughter of Zion' or Israel. Those demons took wives (nations), whom they chose.
In itself, there is something in this theory. We know, partly from Daniel that nations are governed by the powers of darkness. Yet I do not believe that the texts from Genesis 6 should be interpreted like that. It is spiritualizing these texts where no reason is given by the context. The story itself does not fully encompass with this theory.
If so, we are still with the question: Who are the sons of God in Genesis 6? We are going to try to find an answer.
This passage is of course part of a larger whole. The Bible does not begin with chapter 6. Chapters 1 through 5 are there not for nothing.
It all starts with God, who wanted to recreate the chaos into a beautiful planet, full of plants and animals. That chaos was the result of the rebellion of the angel Lucifer who wanted to be equal to the Highest one. On the sixth day of the (re)creation the Lord God created Adam. He created Adam in God's image and in his likeness. In Luke 3:38 we read that Adam was a son of God.
It was not just Adam who was created by God, God created a woman as well, Eve. Adam and Eve were given the mission by God to rule over the earth. God blessed Adam and Eve by making them fertile.
"Be fruitful and increase in number; fill the earth and subdue it".
Adam and Eve lived in paradise. There was also a tree which God had said that they are not allowed to eat from, the tree of knowledge of good and evil.
In the third chapter we are introduced to the subtlety of God's opponent, former angel Lucifer. He spoke through the serpent to Eve and tempted her to eat the fruit that God had forbidden them to eat from. Adam participated.
The result was a break with God. Satan is told that his head will be crushed by the seed of the woman, which is Jesus.
"16 To the woman he said,
"I will greatly increase your pains in childbearing;
with pain you will give birth to children.
Your desire will be for your husband,
and he will rule over you."
When we go beyond the very dramatic story of the fall, we see that God created Adam and Eve and give them first the command to multiply and fill the earth with offspring and subdue it. Then, after the fall, God tells Eve, I will greatly increase your pains in childbearing.
If you've never had anything, then it can not greatly increase.
You don't hear God blaming Adam and Eve that they did not fulfill their mission to multiply, He only says: from now on it will be much harder.
Adam and Eve had children before the fall. How many? We do not know, but there must have been enough people for Cain to find a woman and to be scared by these "others" to be killed.
We can read in chapter 5
"When God created man, he made him in the likeness of God. 2 He created them male and female and blessed them. And when they were created, he called them "man."
3 When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth"
We have just read that Adam was created in the image of God, in Gods likeness. After the fall, Adam was given Set. It is very meaningful that Set was in the likeness of Adam, in his own likeness. A likeness made by man himself by getting separated from God, not a son of God anymore.
Adam was before the fall, a son of God. His children from before the fall are also sons of God!
And those sons of God came in unto the daughters of men after the fall. That was not good according to God. The Spirit of God who was in the sons of God would be removed. They were no longer spiritual, but carnal.
All this took place in the days when the mighty men of old were still on the earth. Do realize that Adam was still alive when Methuselah was born. Adam died when Lamech the father of Noah, was 56 years old. Adam was the ancestor of the mighty. The people of the pre-time (before the flood) listed in chapter 5.