The chief difficulty of interpreting Genesis 6:1-4 lies in the phrase, the sons of God. Who are the sons of God? Are they descendants of Seth? Are they kings? Are they angels?
Now it came about when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, "My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown. (Genesis 6:1-4)To get a better grasp of who the sons of God are, it's important to explore the surrounding context of the passage. Doing so will also make the purpose of this passage clearer, which is just as important, or perhaps even more important than determining the identity of the sons of God.
The first two chapters of Genesis describe the creation of the world. Out of all the created order, man is singled out as unique. Man is not like the common animal; rather, man was made in the image of God. On the sixth day of history God saw all that He had made, and behold, it was very good. (Genesis 1:31) In the beginning, the man, Adam, and the woman, Eve, lived in perfection in the Garden in Eden.
While man was still in the garden, Lucifer, an angel, rebelled against God in heaven. But he was not content to keep his rebellion in heaven, so he came down to the earth. The third chapter of Genesis describes the destruction that this fallen angel caused. Possessing a snake, Satan tempted Eve; she gave into his temptation and ate from the tree. Man fell and was cast out of the garden. It is evident that from the beginning, at least one fallen angel was intimately involved in human affairs. Of course, Satan wasn't the only fallen angel. There were others— and as man's wickedness in the pre-flood world increased, demonic activity must have also increased.
Who Are the Sons of God? (Part 2) - Genesis 6:1-4
The Fall: Satan is Cursed - The Christian Worldview
Summary of Genesis 5
Creation of Man - The Christian Worldview
The Messiah the Jews Expected and the Messiah that Came - The Christian Worldview