There are only two Biblical references to the “Nephilm” (Genesis 6:4 NIV);  Numbers 13:33 NIV). In Genesis 6:4, they are the “giants on earth in those days” before the flood, “the mighty men . . . of renown” (Genesis 6:4 NKJV). The Israelite spies’ “bad report about the land” of Canaan used the term “Nephilm” to describe “people . . . of great size” (Numbers 13:32,33 NIV), from whom the Anakites were claimed to have descended. Upon glimpsing these imposing inhabitants of Canaan, ten of the twelve spies became demoralized and terrified. The “Nephilim” the ten spies spoke of at Numbers 13:33 may have been similar in appearance to the Rephaites, a race of strong, tall people with whom the Anakites are compared in Deuteronomy 2:20,21.

The Nephilim are described in Genesis 6:4 as having been mighty men who lived before the great flood. The inspired writer of Genesis, Moses, linked them to “the sons of God” in terms of being of being their offspring. Three theories have been pro­posed regarding the parentage of these “Nephilim”::

• Some Biblical historians argue that the “sons of God” were righteous men (descen­dants of Seth) who married worldly, female descendants of Cain and thus became de­filed. Their progeny increased in sinfulness until God rectified the worsening situation with the flood. However, this theory does not explain why the word translated “men” in Genesis 6:1 describes all of humanity, while the same word in verse 2 designates only Cain’s line.
• Other scholars argue that “the sons of God” (Genesis 6:2) were kings who took multiple wives in order to build dynasties from their numerous descendants. In several instances ancient Near Eastern documents refer to kings as being the sons of particular gods. Also, Akkadian texts indicate that the Hebrew word translated “men” in Genesis 6:4 could alternatively mean “commoners” in some contexts. This would suggest that the Nephilim were kings who acquired harems, using the daughters of commoners, and sired large families through them. But no other Biblical passages refer to kings in general as “sons of God,” and later kings (such as Sol­omon) who had many wives are not identi­fied as being among the Nephilim.
• Still other scholars believe that the “sons of God” were angels who impregnated hu­man women and sired demigods (beings with more power than humans, but less than gods) who were able to do whatever they pleased on Earth (much like the mythical Greek Titans), prompting God’s determina­tion to destroy humankind to root out the growing evil.

Jesus specified, however, that angels do not marry (Matthew 22:30), and from this it has been argued that they do not procreate. Yet pro­creation by these deviant angels, who took it upon themselves to materialize as human men, is actually aberrant behavior. “The angels too, who did not keep to their own domain but deserted their proper dwelling” (Jude 6 NAB). It may be helpful to note that the phrase “sons of God” as used elsewhere in the Old Testament, and in other ancient Semitic lan­guages, always refers to divine beings (e.g., Job 1:6, where the same Hebrew word is translated “angels” (NIV), in some translations).

Ancient Jewish interpreters unanimously believed the “sons of God” to have been angelic beings, a view which is Biblically correct. This is confirmed by the following New Testament scriptures:

“The spirits in prison, who had once been disobedient while God patiently waited the days of Noah during the building of the ark”—1 Peter 3:19,20 NAB

“The angels too, that did not keep to their own domain but deserted their proper dwelling, he has kept in eternal chains in gloom”—Jude 6 NAB

“God did not spare the angels when they sinned, but condemned them to the chains of Tatarus and handed them over to be kept for judgment”—2 Peter 2:4 NAB

A careful analysis of these scriptures in along with Genesis 6-7 helps us to understand that it was some of the angels in heaven, “the sons of God” who “saw the beautiful women” on earth, became inflamed with wrong desires, and left heaven. They came to earth in materialized human bodies as men, “and took any they wanted as their wives” (Genesis 6:2 NLT).  The apparent result of such unions was that “the Nephilim appeared on the earth in those days” (Genesis 6:4 NAB).  The above scriptures indicate that the “angels” that “sinned”  went back into the spirit world to avoid destruction at the flood, and God put them in “prison,” “the chains of Tartarus,” “in eternal chains in gloom,” gloomily knowing that eventually God will “destroy” them (Luke 4:34). The Nephilim, however, being human, could not go into the spirit world to escape destruction at the flood, and died in the global flood of Noah’s day.

Since the “sons of God” mentioned in Genesis 6 were indeed angels, and the Nephilim were their offspring, how do these facts relate to the “Nephilim” mentioned in Numbers 13? Some think it likely that the word “Nephilim” in this later con­text means something like “giants” or “Ti­tans” (i.e., they think the term was used literally in Genesis 6 but metaphorically in Numbers 13). However,  the inspired record says that “the men Moses had sent to explore the land who returned and made the whole community grumble . . . by spreading a bad report about it—these men who responsible for spreading the bad report about the land were struck down and died of a plague before the LORD” (Numbers 14:36,37 NIV). This indicates that the ten faithless spies grossly exaggerated the truth about the land of Canaan, including their assertion that they “saw the Nephilim there” (Numbers 13:33 NIV).  Since the Nephilim died in the global flood, the spies “bad report” could not possibly have been true. The people in Cannanthe spies saw were just a very tall race of people.

One Source: New International Version Archaeological Study Bible