Genesis 6:1-4 says, “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 which were of old, men of renown.”
The identification of the sons of God referred to in this passage is very difficult. The Bible gives us no definite information on which to reach a firm conclusion. We may never know for certain what is the right answer. However, a plausible guess can be offered. When faced a difficult Biblical question with limited Biblical information to build from, search for an answer that is most reasonable, includes the least amount of unprovable conjecture and best lines up with what is known from other, clear, teachings in the Bible.
After Adam and Eve sinned, they had two sons, Cain and Abel. In a fit of rage, Cain killed Abel and was driven into a distant land called Nod because of his rebellion against God. Cain married and had kids and the kids had kids and so forth. After Cain was driven out, Adam and Eve had another son named Seth. Seth and his family apparently stayed in the region with Adam and Eve. A major part of Seth’s family remained faithful and obedient to God while Cain’s family continued in rebellion against God.
During the days of Noah the entire world was divided into two population groups, those who worshiped God and those who didn’t. It is my opinion that the males of Seth’s family who continued to worship God were called the sons of God. Adam lived for nearly a thousand years after being created by God in the garden of Eden. Those of Adam’s family that stayed with him and continued to worship God might very reasonably be called the children of God. Those who did not might very well have been called the children of men.
What about the phrase, “the same became mighty men . . . men of renown”? Some have connected the statement about giants in the earth with the men of renown born as a result of the coupling of the sons of God and daughters of men. However, Genesis 6:4 does not say that. The verse says there were giants on the earth and also the descendants of the sons of God and daughters of men became men of renown. The men of renown need not be gigantic beings or some sort of demigod. The mighty men are nothing more than the great men of the pre-flood days, probably those mentioned in Genesis 4.
I think the sons of God were the obedient descendants of Seth. As the world’s population increased, the descendants of Seth and the descendants of Cain began to mingle and intermarry, thus the sons of God married the daughters of men.