“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
Well, that may have been a bit unexpected. It may be tempting here by some to attribute this portion of the text (that is, the Nephilim) to some carryover from Greek mythology or similar. However the Nephilim are mentioned again, in Numbers 13. It is also entirely likely that Goliath the giant, from Gath of the Philistines was a remnant of the offspring from the Nephilim. The point being that if you just discredit this portion of scripture, it begins to affect other passages, even to the point of possibly discrediting the story of David and Goliath.
So what were they? According to Alter’s commentary, the Hebrew would translate to “the fallen ones”. They are referred to as the “sons of God”, a phrase which in the book of Job appears to be referring to Angels (see Job 1:6, 2:1 and 38:7). We know by Revelation 12:9 that angels were thrown down to earth.
It is true that Jesus made the statement “angels are neither married nor given in marriage…” (see Matthew 22:30) We may infer from Jesus’ statement that angels do not have the ability to reproduce, though that was not specifically said. In scripture we only specifically know of Cheribum and Serphim, so perhaps the Nephilim are a third type of creature altogether.
An alternative answer is that the “sons of God” were actually the godly descendants of Seth, who mixed with Cain’s descendants. My position would be that Genesis 5 already sums up the remainder of Cain’s descendants. Additionally the text tells us that these Nephilim were also on the earth after the time of the flood. As humans go, we only know of eight (Noah and his wife, his three sons and their three wives) that survived (Gen. 7:13). If the Nephilim survived the flood, well, they were not human. I think this theory of the line of Seth representing the sons of God comes about because the idea of supernatural creatures cohabiting with human women freaks us out, and the text doesn’t go out of its way to give us a thorough enough explanation. But does that make it untrue? No it does not. Lastly, simply mating Seth’s line with Cain’s line would give us no reason to believe that giants would result.
Og, the king of Bashan, was said to be of the Rephaim, ultimately from the line of the giants. His bed, according to Deuteronomy 3:11, was 13 feet by 6 feet in size. Giants are mentioned elsewhere in scripture as well. So regardless of what you believe about the origin of the Nephilim, there were ultimately giants in the land to contend with.