"Nephilim are beings who appear in the Hebrew Bible, specifically in the Book of Genesis, and are also mentioned in other Biblical texts and in some non-canonical Jewish writings. Genesis Chapter 6, verses 1 through 4 describe the origin of the Nephilim. According to the New American Bible, the Nephilim appear as part of the "increasing wickedness of mankind".Genesis 6:4 implies that the Nephilim have inhabited the earth in at least two different time periods--in antediluvian times "and afterward." If the Nephilim were unearthly beings, it is possible that the "giants of Canaan" in Book of Numbers 13:33 were the direct descendants of the antediluvian Nephilim. An alternate possibility is that the term Nephilim (Hebrew: נְפִילִים) is a generic term for "giants" in general, which is consistent with the Septuagint and Vulgate translations of the word. Some expositors believe it may refer more to the ferocity and strength of the people who are referred to, rather than their physical height, though in the Book of Numbers, intentional stress on height is apparent.
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’.
A long-held view among Christians is that the sons of God are not the Nephilim, but the righteous descendants of Seth, while the Nephilim are the unrighteous descendants of Cain. This view dates to at least the 3rd century AD, with references in Sextus Julius Africanus, as well as throughout the Clementine literature."