Disclaimer: Long post but I hope it will have all the information to explain. This is just a copy/paste from GotQuestons.org. I will provide the links.
Question: "Who were the sons of God and daughters of men in Genesis 6:1-4?"
Answer: Genesis 6:1-4 refers to the sons of God and the daughters of men. There have been several suggestions as to who the sons of God were and why the children they had with daughters of men grew into a race of giants (that is what the word
Nephilim seems to indicate).
The three primary views on the identity of the sons of God are 1) they were fallen angels, 2) they were powerful human rulers, or 3) they were godly descendants of Seth intermarrying with wicked descendants of Cain. Giving weight to the first theory is the fact that in the Old Testament the phrase “sons of God” always refers to angels (
Job 1:6;
2:1;
38:7). A potential problem with this is in
Matthew 22:30, which indicates that angels do not marry. The Bible gives us no reason to believe that angels have a gender or are able to reproduce. The other two views do not present this problem.
The weakness of views 2) and 3) is that ordinary human males marrying ordinary human females does not account for why the offspring were “giants” or “heroes of old, men of renown.” Further, why would God decide to bring the flood on the earth (
Genesis 6:5-7) when God had never forbade powerful human males or descendants of Seth to marry ordinary human females or descendants of Cain? The oncoming judgment of
Genesis 6:5-7 is linked to what took place in
Genesis 6:1-4. Only the obscene, perverse marriage of fallen angels with human females would seem to justify such a harsh judgment.
As previously noted, the weakness of the first view is that
Matthew 22:30 declares, “At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven.” However, the text does not say “angels are not able to marry.” Rather, it indicates only that angels do not marry. Second,
Matthew 22:30 is referring to the “angels in heaven.” It is not referring to fallen angels, who do not care about God’s created order and actively seek ways to disrupt God’s plan. The fact that God’s holy angels do not marry or engage in sexual relations does not mean the same is true of Satan and his demons.
View 1) is the most likely position. Yes, it is an interesting “contradiction” to say that angels are sexless and then to say that the “sons of God” were fallen angels who procreated with human females. However, while angels are spiritual beings (
Hebrews 1:14), they can appear in human, physical form (
Mark 16:5). The men of Sodom and Gomorrah wanted to have sex with the two angels who were with Lot (
Genesis 19:1-5). It is plausible that angels are capable of taking on human form, even to the point of replicating human sexuality and possibly even reproduction. Why do the fallen angels not do this more often? It seems that God imprisoned the fallen angels who committed this evil sin, so that the other fallen angels would not do the same (as described in
Jude 6). Earlier Hebrew interpreters and apocryphal and pseudopigraphal writings are unanimous in holding to the view that fallen angels are the “sons of God” mentioned in
Genesis 6:1-4. This by no means closes the debate. However, the view that
Genesis 6:1-4 involves fallen angels mating with human females has a strong contextual, grammatical, and historical basis.
http://www.gotquestions.org/sons-of-God.html
Question: "Who / what were the Nephilim?"
Answer: The Nephilim (“fallen ones, giants”) were the offspring of sexual relationships between the sons of God and daughters of men in
Genesis 6:1–4. There is much debate as to the identity of the “sons of God.” It is our opinion that the “sons of God” were fallen angels (demons) who mated with human females or possessed human males who then mated with human females. These unions resulted in offspring, the Nephilim, who were “heroes of old, men of renown” (
Genesis 6:4).
Why would the demons do such a thing? The Bible does not specifically give us the answer. Demons are evil, twisted beings—so nothing they do should surprise us. As to a distinct motivation, one speculation is that the demons were attempting to pollute the human bloodline in order to prevent the coming of the Messiah. God had promised that the Messiah would one day crush the head of the serpent, Satan (
Genesis 3:15). The demons in
Genesis 6 were possibly attempting to prevent the crushing of the serpent and make it impossible for a sinless “seed of the woman” to be born. Again, this is not a specifically biblical answer, but it is biblically plausible.
What were the Nephilim? According to Hebraic and other legends (the Book of Enoch and other non-biblical writings), they were a race of giants and super-heroes who did acts of great evil. Their great size and power likely came from the mixture of demonic “DNA” with human genetics. According to the movie
Noah, starring Russell Crowe (reviewed by us
here), the Nephilim were fallen angels encased in rock. All that the Bible directly says about them is that they were “heroes of old, men of renown” (
Genesis 6:4). The Nephilim were not aliens, angels, “Watchers,” or rock monsters; they were literal, physical beings produced from the union of the sons of God and the daughters of men (
Genesis 6:1–4).
What happened to the Nephilim? The Nephilim were one of the primary reasons for the great flood in Noah’s time. Immediately after the mention of Nephilim, God’s Word says, “The LORD saw how great man’s wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. The LORD was grieved that he had made man on the earth, and his heart was filled with pain. So the LORD said, ‘I will wipe mankind, whom I have created, from the face of the earth—men and animals, and creatures that move along the ground, and birds of the air—for I am grieved that I have made them’” (
Genesis 6:5–7). God proceeded to flood the entire earth, killing everyone and everything other than Noah, his family, and the animals on the ark. All else perished, including the Nephilim (
Genesis 6:11–22).
Were there Nephilim after the flood?
Genesis 6:4 tells us, “The Nephilim were on the earth in those days—and also afterward.” It seems that the demons repeated their sin sometime after the flood as well. However, it likely took place to a much lesser extent than it did prior to the flood. When the Israelites spied out the land of Canaan, they reported back to Moses: “We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them” (
Numbers 13:33). This passage does not say the Nephilim were genuinely there, only that the spies
thought they saw the Nephilim. It is more likely that the spies witnessed very large people in Canaan and in their fear believed them to be the Nephilim. Or it is possible that after the flood the demons again mated with human females, producing more Nephilim. It is even possible that some traits of the Nephilim were passed on through the heredity of one of Noah’s daughters-in-law. Whatever the case, these “giants” were destroyed by the Israelites during their invasion of Canaan (
Joshua 11:21–22) and later in their history (
Deuteronomy 3:11;
1 Samuel 17).
What prevents the demons from producing more Nephilim today? It seems that God put an end to demons mating with humans by placing all the demons who committed such an act in isolation. Jude verse 6 tells us, “The angels who did not keep their positions of authority but abandoned their own home—these he has kept in darkness, bound with everlasting chains for judgment on the great Day.” Obviously, not all demons are in “prison” today, so there must have been a group of demons who committed further grievous sin beyond the original fall. Presumably, the demons who mated with human females are the ones who are “bound with everlasting chains.” This would prevent any more demons from attempting such sin.
http://www.gotquestions.org/Nephilim.html
Question: "Are the demons the disembodied spirits of the Nephilim?"
Answer: With the understanding that the sons of God were the fallen angels, and that the Nephilim were the hybrid offspring of the union between the fallen angels and human women, the question then arises, What happened to the spirits of the Nephilim after they were killed, whether by the flood, or in the case of the possible post-flood Nephilim (
Genesis 6:4;
Numbers 13:33), after the flood?
Some speculate that the disembodied spirits of the Nephilim remained on the earth and became what we now refer to as demons. The presumption is that, as angelic-human hybrids, the spirits of the Nephilim would have been different from the human soul-spirit, having the ability to remain present in this world despite no longer having a physical body. This would possibly explain the desire the demons have to possess human beings, thus gaining control over a physical body. This would also make some sense from the perspective of the fallen angels, who are outnumbered 2-1 by the holy angels, giving them a good reason to seek to increase their ranks.
The Nephilim explanation for the origin of the demons is partly the result of a misunderstanding of who exactly are the “
spirits in prison” in
1 Peter 3:19 (see also
Jude 6). Many misunderstand the “spirits in prison” to be all of the fallen angels who rebelled against God. If all of the fallen angels are imprisoned, then there must be an alternate explanation for the existence of demons; thus, the need for the Nephilim explanation. However, clearly, not all of the fallen angels are imprisoned. Satan, the leader of the angelic rebellion against God, is not imprisoned. Why would God allow the rebel leader to remain free but then confine the angels who followed Satan in the rebellion? No, it makes more sense to understand the “spirits in prison” as the fallen angels who participated in an additional rebellion, viz., the sons-of-God/daughters–of-men incident. The fallen angels who mated with human females are the ones who are imprisoned. There is no solid biblical reason to reject the idea that the demons are the same beings as the fallen angels.
The idea that the demons are the disembodied spirits of the Nephilim is also drawn from the
book of Enoch, which goes into great detail regarding the Nephilim. We have to remember that, while the book of Enoch contains some truth (
Jude 14), it is not the inspired, inerrant, and authoritative Word of God. We should never base a belief exclusively, or even primarily, on extra-biblical literature. So, with no need to explain the existence of demons outside of the fallen angels, and with no clear evidence in Scripture for the spirits of the Nephilim continuing on Earth, there is no solid basis on which to identify the demons with the spirits of the Nephilim. While the idea is possible, it cannot be derived explicitly from Scripture, and therefore should not be considered the best explanation of the origin of the demons.
http://www.gotquestions.org/Nephilim-demons.html
