What are they?
Most consider them to be the offspring of the “sons of God” and the “daughters of men” before the Flood. The name is also used in reference to giants who inhabited Canaan at the time of the Israelite conquest of Canaan according to Numbers 13:33.
The majority of ancient biblical versions, interpret Nephilim to mean “giants”. Many interpretations are based on the assumption that the word is a derivative of Hebrew verbal root n-ph-l “fall”. Those interpretations include “those that cause others to fall down”, “ones who have fallen”, equivalent grammatically to “one who is appointed” (i.e., overseer), asir, “one who is bound”, (i.e., prisoner), or simply “fallen” “apostates”. Symmachus translates it as “the violent ones” and Aquila’s translation has been interpreted to mean either “the fallen ones” or “the ones falling [upon their enemies]”. According to the Brown-Driver-Briggs Lexicon, the basic etymology of the word Nephilim is “dub[ious]”, and various suggested interpretations are “all very precarious”. What all the interpretations seem to agree upon is that the Nephilim were superior to average humans, and dangerous.
The New American Bible commentary draws a parallel to the Epistle of Jude and the statements set forth in Genesis, suggesting that the Epistle refers implicitly to the paternity of nephilim as heavenly beings who came to earth and had sexual intercourse with women. The footnotes of the Jerusalem Bible suggest that the biblical author intended the nephilim to be an “anecdote of a superhuman race”.
Where do they come from?
While the consensus seems to be that Nephilim are the offspring of angels and men, some early texts suggest that they are actually the offspring of Seth and the “daughters of Cain”. Once such text is the Qumran (Dead Sea Scroll) fragment 4Q417 which contains the earliest known reference to the phrase “children of Seth“, stating that God has condemned them for their rebellion. Other early references to the offspring of Seth rebelling from God and mingling with the daughters of Cain, are found in rabbi Shimon bar Yochai, Augustine of Hippo, Julius Africanus, and the Letters attributed to St. Clement. It is also the view expressed in the modern Ethiopian Orthodox Bible.
Nevertheless, most agree that the Nephilim were indeed the offspring of angels: A number of early sources refer to the “sons of heaven” as angels. The earliest such references to this also appear in the Dead Sea Scrolls, Greek, and Aramaic Enochic literature, and in certain Ge’ez manuscripts of 1 Enoch and Jubilees used by western scholars in modern editions of the Old Testament Pseudepigrapha. The earliest statement in a secondary commentary explicitly interpreting this to mean that angelic beings mated with humans can be traced to the rabbinical Targum Pseudo-Jonathan and it has since become especially commonplace in modern-day Christian commentaries.
Some Christian commentators have argued against this view, citing Jesus’s statement that angels do not marry. Others believe that Jesus was only referring to angels in heaven.
Evidence cited in favor of the “fallen angels” interpretation includes the fact that the phrase “the sons of God” (Hebrew, בְּנֵי הָֽאֱלֹהִים; literally “sons of the gods”) is used twice outside of Genesis chapter 6, in the Book of Job (1:6 and 2:1) where the phrase explicitly references angels. The Septuagint manuscript Codex Alexandrinus reading of Genesis 6:2 renders this phrase as “the angels of God” while Codex Vaticanus reads “sons”.