The passage in Genesis 6 speaking of the sons of God, the daughters of men, and the nephilim is arguably one of the more befuddling passages in Genesis, if not in the whole Bible. And no shortage of ink has been used over the centuries trying to make sense of it. In such a short passage, seemingly so much is said, but almost no answers are provided. And people have been fascinated and bewildered by it since before the time of Christ.
We have, for example, the books which were later compiled into the singular work now known as 1 Enoch, which were written between around 300 BC and 100 AD, and there is a rather robust and fantastic exploration of this. Where we have angels known as “Watchers” who were seduced by the beauty of human women, and so became disobedient, they fell, and then taught human beings all manner of things and also siring offspring with human women to produce a race of “giants”. And this becomes, in the Enochian narrative, a principle reason for why the flood was necessary.
This is why we see in 2nd Temple Jewish works like the Enochian texts, and even among some early Christian writers of the first couple of centuries (who were avid readers of the Enochian texts, even though “1 Enoch” was never embraced as canonical Scripture by the ancient Church*)
In time, however, both Jewish and Christian authorities came to embrace a less fantastical interpretation of the Genesis 6 passage, and the general consensus both within historic Christian circles and historic Jewish circles is that the things described in the text are much more mundane–“children of God” isn’t an oblique reference to celestial beings (aka angels), but a group of humans, perhaps those descended from Seth; while “daughters of men” came to be understood as other humans, perhaps the descendants of Cain.
In modern times things have often become far more elaborate, with not only fantastical views echoing the ancient Enochian fiction; but all manner of ideas which look a lot like tinfoil hattery.
I think it is worth noting a couple things. The text mentions “sons of God” and “daughters of men” coming together because the “sons of God” found “the daughters of men” to be beautiful. It does not, however, mention that the resulting offspring of this union were the nephilim. It says “The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them.”. Notice the phrasing, the nephilim were on the earth in those days and also afterward when; not as a result of. Though it is certainly tantalizing to speculate or imagine that perhaps this union of the two groups produced this mysterious “nephilim” (Hebrew for “fallen ones”).
The passage then continues “These were the mighty men who were of old, the men of renown.” Well, what the heck does that mean? Is this referring to the nephilim or to the children born of the sons of God and daughters of men? Are the nephilim actually the offspring? Yet it says they were on the earth when the sons of God took the daughters of men as wives and had children with them–that suggests the nephilim are already there when this happened.
The “mighty men of old, the men of renown” seems to suggest un-mentioned heroic figures, that is we don’t know who these men were, but they seem to have a legendary status of some kind, they are ancient. In the Hebrew of the text they are gibborim aser me-olam “mighty ones which [are] from old”, anshy ha-shem “men of the renown”/“mortals of renown”. Ancient, famous, but we aren’t told who/what they are, though they appear to be the offspring of the sons of God and daughters of men as the text connects the offspring to them with the word הֵמָּה (“these”/“they”).
So, again, what we have is a very short text, without virtually any answers to any questions we might pose to this passage. And this isn’t aided by the fact that the LXX would go on to translate both nephilim and gibborim with the word gigantes, “giants”. Which along with the nephilimic references later, including the Israelite spies who went into Canaan and returned to report that the inhabitants were as giants, and then we have the case of Goliath of Gath described as a giant (of course, the Masoretic version of Goliath’s height is quite fantastical, compared to the much older version of Goliath’s height given in e.g. the LXX and DSS, reflecting a far more sensible height of about 6.5-7 ft as opposed to nearly 10 ft. which would have still been massive compared to the average male height of 5’6" at the time and place). Though mention of “giants” and the tantalizing idea of “sons of God” and “daughters of men” producing offspring, the mention of a nebulous group called “fallen ones”, and of legendary–but not told to us in the text–people all creates a temptation to speculate endlessly about what it all means. At the end of the day, however, we have a lot of questions, but the text doesn’t give up its secrets. And perhaps the most prudent thing we can do is allow Scripture to say what it says, and simply say we don’t really know what it all means. It almost certainly made sense to the original readers, but sometimes the Bible just isn’t clear to us modern readers. And maybe that’s okay.
*Some ancient fathers, such as Tertullian (though Tertullian is himself a controversial figure due to his having abandoned orthodox Christianity and converting to the Montanist cult) seem to give Enoch scriptural status, but this is never reflected in the general consensus of the Church; and then there is the peculiar case of the Ethiopian Orthodox Church which, even today, has a biblical Canon with a far more extensive canonical list than any of the other historic churches which embraces 1 Enoch, Jubilees, and many others. However, again, speaking of the general and broad consensus of the Christian Church, Enoch has never been received or accepted as Scripture.