There are two references in Bible that tell of some manner of creatures called “Nephilim”. The first is the 6th chapter of Genesis right before the Flood, which also contains a reference to the “sons of God”:

“(v.1) When people began to multiply on the face of the ground, and daughters were born to them, (v.2) the sons of God saw that they were fair; and they took wives for themselves of all that they chose…(v.4) The Nephilim were on the earth in those days—and also afterward—when the sons of God went in to the daughters of humans, who bore children to them. These were the heroes that were of old, warriors of renown.”

Do Moses’ Spies really see a Race of Giants?

The second reference is post-Flood. Given that it is said elsewhere that the whole human race is repopulated solely from Noah and his sons, there seems to be case for asserting that these are not the original Nephilim, unless they survived the Flood at the time in an unincluded caveat. However it must also be noted that this report is contradicted by Caleb and Joshua and it is highly likely that it is a report that is being exaggerated by the spies who are sent to the land by Moses and Aaron who did not have the stomach for attacking these powerful cities and were truly intimidated by them. Further, we do not in fact see any encounters between the Israelites and any supernaturally giant races when they do enter the land (although there are some large individual enemy warriors that are mentioned at the time of David):

“And there we saw the Nephilim, the sons of Anak, who come of the Nephilim; and we were in our own sight as grasshoppers, and so we were in their sight” (Numbers 13:32-33)

There is a disputed connection between the two manner of creatures: “sons of God” and “Nephilim- the heroes of old/ the warriors of renown”, and as to whether the latter is the product of the union of the former with human women. The passage seems to lends credence to both readings, since it does not state that the Nephilim were themselves the products of that union, although this admittedly seems to be strongly suggested by it.

Are the sons of God Angels?

Here the point in favor of the sons of God being angelic beings is that the term “sons of God” itself is used in other places, as in the Book of Job and the Psalms as referring to angels, and also that this seems to be the manner in which the incident is referenced in the the New Testament.

Jude 1:6 “ And the angels who did not keep their own position, but left their proper dwelling, he has kept in eternal chains in deepest darkness for the judgment of the great day. Likewise, Sodom and Gomorrah and the surrounding cities, which, in the same manner as they, indulged in sexual immorality and pursued unnatural lust, serve as an example by undergoing a punishment of eternal fire. Yet in the same way these dreamers also defile the flesh, reject authority, and slander the glorious ones. But when the archangel Michael contended with the devil and disputed about the body of Moses, he did not dare to bring a condemnation of slander against him, but said, “The Lord rebuke you!” 

It can be seen that the passage seems to suggest that the people of Sodom and Gomorrah “in the same manner as they, in the same manner as they, indulged in sexual immorality and pursued unnatural lust” seems to be referring to those angels actually committing sexual offence too, “leaving their proper station” and “pursued unnatural lust”. However one notes that the author of Jude raises up this incident in order only to condemn it, that it is certainly not the natural or God-intended manner of living for angels here. 2 Peter 2:4 also refers to the punishment of angels who “sinned” without specifying the nature of the offence.

Are they Earthly Kings?

The alternative, that the “sons of God” are referring to earthly kings who could possibly have taken such titles, as is also seen later in the Bible, and may have been also the practice in those times. This view is well-argued for by Michael Jones of Inspiring Philosophy in the video supplied. Since the royalty might be expected to possibly be of good genetic stock and also have access to good nutrition and training as compared to the rest of the populace, this might explain why their offspring went on to become “warriors of renown”.

How might Angels effect Human Reproduction?

Jimmy Akin points out in his video that the angels are telekinetic, i.e. they are able to influence matter and volitionally produce physical effects- they can spread plagues, and take lives as in battles fought for the prophets and in the case of the Egyptian first-born sons, and other examples in the Bible. Akin points out that since they can influence matter at the microscopic level of micro-organisms, there should be no reason to believe they could not also engineer DNA, for which even humans are gradually gaining the capability. This means that in order to produce offspring in a woman, they should possibly be able to create a copy of chromosomes to complement the copy in the woman’s egg. All this certainly sounds very uncomfortable and certainly too close to “playing God” for comfort, but we should probably treat these angels as “fallen”, those that fell to human desire. Again this is hard to explain, angels are not inherently given to human desire as we further discuss later.

In the same way, genetic engineering is very uncomfortable so much so that ever secular governments are often loathe to grant such projects ethical approval. Put it like this- angels are spiritual beings in Christian belief. Could it be the case that they assembled DNA molecules either from scratch or from recombination from pre-existing cells. In that case it could be that these children were engineered to have great stature, like a modern-day basket-ball player as well as great strength and perhaps a belligerent/risk-taking disposition. It is thought that the average height of the ancients was just above 5 feet, so anyone much taller would seem like a giant, and also be more likely to get chosen as a leader due to physical prowess.

Finally, were it truly the case that angels raised offspring with humans, what could possibly be the motivation for having such children which were not in anyway “theirs”- that is- I cannot see how such an angel could have a sense of belonging to the result of such a pregnancy. Or perhaps it did? Or perhaps it was a means of convincing the women involved that they had been pregnant by them. The biblical text does not state the motivations. In any case we are also not told of angels and human women actually living together as husband and wife in co-habiting relationships, and so the encounters really seem more akin to demonic possessions of the woman’s body.

An Unnatural Act, not the Norm

The incident of the Nephilim and their relation to the “sons of God” is mysterious and obscure and confined to three OT verses. Were it true that angels managed to interact with the human genome, this is an obviously unnatural act. Firstly, a species is either one species or the other, not a mixture of two, and secondly, it would seem highly counter-intuitive that angels possess DNA, that they might use it in such an interaction. At the most we might expect that they sinfully caused some sort of physical effect through a process akin to genetic engineering that humans themselves are becoming increasingly and sinfully capable of themselves, in the process “creating” chimeric individuals and rightly being punished for it as related in Jude, if indeed this is referring to the same incident.

Sexual Desire is not a Feature of Angelic Species

The sexual desire itself is a natural feature of a species which requires to reproduce sexually for its survival, there should be no reason to suppose that angels would have any such aspect of desiring apart from projecting our own desires onto them, further, there would be no reason to suppose that such species would even be male and female, as indeed the angels are never described. The manifest purpose of the angels is to glorify God and to serve him, it would be unlikely that they be taken up instead in pursuing amorous relations with other angels instead.

The Possibility that Apocryphal Accounts are used for Teaching

Finally we can see that the writer of Jude is possibly accessing apocryphal accounts anyway, as what immediately follows in the passage is a dispute between Michael and the Devil over the “bones of Moses” which is unlikely to be a real event. So also the writers of the Old Testament might in some interpretations be referencing legendary material for teaching, if indeed it is to be held that accounts like that of the Flood are to be taken as legendary. In that case Jesus himself in referencing this material with relation to the Flood has a similar purpose, that is, of referencing local legends purely for the purpose of teaching.

Conclusion, and the Judaic view

The incident of the Nephilim is a one-off in the Bible rather than a foundational incident in Christian theology and it is hardly the case that we are to expect angelic unions with women as an ongoing occurrence or a normative expectation. In fact, since that one mention, there has not been any such assertion even in Judaism, leave alone Christian history. The Jewish prophets starting from Moses never speak about spirit beings physically interacting with humans. One would think that if this was something of a current reality of all the people persons like Moses, who was given to see God himself face to face could see also, other other spiritual giants like Samuel, Elijah and King David among many others.

In other words, the incident, to say the least is obscure even to the Jews, who do not dwell on it in their writings. Here’s a couple of short articles from well-known websites https://www.jewishvirtuallibrary.org/nephilim and https://www.chabad.org/parshah/article_cdo/aid/1987422/jewish/Nephilim-Fallen-Angels-Giants-or-Men.htm, both of which combine some mythologizing with speculation and alternative explanations.

We deal with the Flood narrative, and the possibility of metaphorical aspects of it here The Flood Account – Literal or Metaphor?