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 contention that the “sons of God” were fallen angels (demons) who mated with human females and/or possessed human males and then mated with human females. These unions resulted in offspring, the Nephilim, that 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. The demons are evil, twisted beings—so nothing they do should surprise us. As to a distinct motivation, the best speculation is that the demons were attempting to pollute the human blood line in order to prevent the coming of the Messiah, Jesus Christ. God had promised that a Messiah would come from the line of Eve (Genesis 3:15) who would crush the head of the serpent, Satan. So, the demons were possibly attempting to prevent this by polluting the human bloodline, making it impossible for a sinless Messiah to one day be born. Again, this is not a specifically biblical answer, but it is plausible and not in contradiction to anything the Bible teaches.
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. 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, they were literal, physical beings produced from the union of the sons of God and 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 Nephilim are mentioned, God’s Word tells us this: “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). So, God proceeded to flood the entire earth, killing everyone and everything (including the Nephilim) other than Noah and his family and the animals on the ark (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). Now, this passage does not specifically say that 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 mistakenly believed them to be the Nephilim. Or, it is possible that after the flood the demons again mated with human females, producing more Nephilim. 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 the Abyss. Jude verse 6 tells us, “And 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 of the 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 an act.