The Nephilim “were giants – the same race that terrified Moses’ spies (Numbers 13:33). They were given this title from the root (Hebrew letters)* to fall*, because they fell and caused others to fall (Rashi), through their egregious sinfulness (Gur Aryeh). Alternatively, they were so called because the hearts of those who saw them fell in amazement at their size (Ibn Ezra).” (Stone Edition Chumash, Book of Genesis, verse 4 commentary, p. 27):
“The Nephilim were on the earth in those days and also afterward, for the sons of the rulers would consort with the daughters of man, who would bear children to them. They were the mighty who, from old, were men of fame.” (Genesis 6:4, Ramban commentary on the Torah, p. 167).
Jewish Sage Ramban elucidates:
"(In those days and also afterward):
(What periods of time are meant by in those days and afterward? Ramban explains, beginning by citing Rashi):
This is a quote from Rashi:
(In those days) — in the days of the generation of Enosh. And also afterward – though they saw the destruction of the generation of Enosh, when the ocean overflowed and inundated a third of the world (Note: as punishment for worshiping idolatry) – and they were not humbled to learn from them. And Rabbi Avraham Ibn Ezra explains also afterward to mean “also after the Flood.” For, he continues, the “children of the giants” were from the family of the “children of the rulers”.
(If, as Ibn Ezra maintains, the Nephilim who lived after the Flood were descended from the Nephilim of our verse (who lived* in those days*, i.e., before the Flood,) how did this family survive the Flood? Ramban presents two possibilities):
If so, either the wives of Noah’s sons were of (the Nephilim’s) offspring and were similar to them in stature, and the family survived through them, or else he (Ibn Ezra) agrees with the Sages’ statement (Niddah 61a) that expounds concerning Genesis 14:13 that the giant Og survived the flood, but he goes further than that, asserting that other Nephilim also survived along with (Og).
(Ramban now presents an entirely new approach to this section):
The soundest explanation in my view is that Adam and his wife were called “children of God,” ---- because they were His handiwork, and He was their father, as it were, for they had no other father. And (Adam) begot many children, as it is written (above 5:4) and he begot sons and daughters.
Now, these were the first ever to be born to a father and mother, possessed great perfection in height and strength, for they were born in the image of their father, who was the epitome of human perfection, as is written concerning Seth: and he begot in his likeness and his image (above 5:3).
(Ramban offers a second possible explanation as to why the term children of God was applied to the first human beings):
It is also plausible that all sons of the first people in the world – Adam, Seth, Enosh – were called “children of God,” because these three men were in the image of God, meaning that they were gifted with physical perfection. But then the practice of idol worship began, and as a result of this a weakness and frailty began to affect people.
(Ramban shows that there is an opinion in the Midrash that corroborates this interpretation):
And similarly they said in Bereishis Rabbah (24:6): This is the account of the descendants of man (5:1) – The implication is that the previous people discussed just before this verse (Adam, Seth, and Enosh, mentioned at the end of Ch. 4) were not descendants of man. So what were they? Godly beings. (The students) asked this question before Abba Kohen Bardela: Why does Scripture, at the end of Ch. 4, tell us about Adam, Seth, and Enosh, and then fall silent and not discuss the subsequent descendants, as it did in Ch. 5? He answered them: Until this point (until Enosh) people were in God’s image and likeness; from this point on, beginning with Kenan, they acted in a way that angered God.
(Ramban resumes his explanation of our passage):
Now, when man began to proliferate, and they had many daughters, these sons of the first men (i.e., the “children of God”) were still in their full strength, for they did not experience the weakening of physical constitution that took place after Enosh. And because of their great desire they would choose the women who were the best, possessors of great height and health. And (Scripture) relates at first (v. 2) that they took them for wives unjustly, i.e., against their will, – and after this it relates (v. 4) that they also had illicit relations in secret with the daughters of man, i.e., women who were not on that high level of physical perfection. — But this matter of their secret promiscuity did not become known until (these) ordinary women bore them sons – and people recognized that (these) offspring were not children of ordinary men, but were born from these “children of God,” because they were huge. However, the children were inferior to their fathers in height and strength…These children were known as the mighty ones, in comparison to other, ordinary people."
