City of God - Book 15, Sections 22 and 23
Much has been written about the 'Nephilim' (or 'giants') alluded to in Scripture. In pop culture, the word is used to describe the superhuman offspring of a human and an angel (or a fallen angel), and there are many Christians who hold this interpretation.
Some of the most bizarre theories I have encountered include the idea that some of these Nephilim or giants were the size of mountains --- and some mountains are in fact their petrified remains. I've even come across people like Gary Wayne (author of The Genesis 6 Conspiracy) and his cronies who think it is a worthwhile expenditure of time to speculate on late-night YouTube streams about how a mountain-sized giant could copulate with an ordinary human woman.
Regarding the following passage, the phrase 'sons of God' is sometimes used as a euphemism for 'angels' in Scripture.
Genesis 6 :
And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
This passage and a few others easily lend themselves to fantastical interpretations. Augustine, however, strongly rejects the fantastical interpretations and the possibility of human-angel hybrids. He opts for a more mundane interpretation that meshes with the themes of the surrounding passages. In his view, this story is simply about how the Sethites began intermarrying with the Cainites.
Augustine points out that the phrase ‘sons of God’ does not necessarily mean ‘angels’ and that elsewhere in Scripture even the word ‘angel’ is sometimes used to refer to godly but otherwise ordinary men. So the stories in the Bible like Genesis 6 that seem to be talking about angels marrying human women may not actually mean what they sound like.
In Augustine’s view, there were probably many giants in the antediluvian world and when the Sons of Seth (the antediluvian Church) began intermarrying with the Sons of Cain (the ungodly ‘City of Man’), many of their children just so also happened to be giants (or ‘Nephilim’) but in the big picture this is not anything particularly remarkable, and there is no reason to suspect supernatural hanky-panky.
Interestingly, Augustine makes a distinction between lesser devilish entities (like ‘incubi’ and ‘Duses’) and fallen angels. He does not deny the evidence that a lesser spirit might (vainly) attempt to copulate with a human woman, but he can find no scriptural evidence that a fallen angel could be motivated to attempt such a thing.
Augustine rejects what certain apocryphal books have to say about the Nephilim, but then appeals to a quotation from the deuterocanonical Book of Baruch. (The deuterocanonical books are a set of apocryphal writings that are rejected by Protestants but not by Catholics).
[…] Scripture testifies that angels have appeared to men in such bodies as could not only be seen, but also touched. There is, too, a very general rumour, […], that sylvans and fauns, who are commonly called "incubi," had often made wicked assaults upon women […]; and that certain devils, called Duses by the Gauls, are constantly attempting and effecting this impurity is so generally affirmed, that it were impudent to deny it. From these assertions, indeed, I dare not determine whether there be some spirits embodied in an aerial substance (for this element, even when agitated by a fan, is sensibly felt by the body), and who are capable of lust and of mingling sensibly with women; but certainly I could by no means believe that God's holy angels could at that time have so fallen, nor can I think that it is of them the Apostle Peter said, "For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment." I think he rather speaks of those who first apostatized from God, along with their chief the devil, who enviously deceived the first man under the form of a serpent. But the same holy Scripture affords the most ample testimony that even godly men have been called angels; for of John it is written: "Behold, I send my messenger (angel) before Thy face, who shall prepare Thy way." And the prophet Malachi, by a peculiar grace specially communicated to him, was called an angel.
But some are moved by the fact that we have read that the fruit of the connection between those who are called angels of God and the women they loved were not men like our own breed, but giants; just as if there were not born even in our own time (as I have mentioned above) men of much greater size than the ordinary stature. […]Giants therefore might well be born, even before the sons of God, who are also called angels of God, formed a connection with the daughters of men, or of those living according to men, that is to say, before the sons of Seth formed a connection with the daughters of Cain. […]These words of the divine book sufficiently indicate that already there were giants in the earth in those days, in which the sons of God took wives of the children of men, when they loved them because they were [attractive]. But after this connection had been formed, then too were giants born. […]
But that those angels were not angels in the sense of not being men, as some suppose, Scripture itself decides, which unambiguously declares that they were men. […]
Let us omit, then, the fables of those scriptures which are called apocryphal, […]. There is therefore no doubt that, according to the Hebrew and Christian canonical Scriptures, there were many giants before the deluge, and that these were citizens of the earthly society of men, and that the sons of God, who were according to the flesh the sons of Seth, sunk into this community when they forsook righteousness. Nor need we wonder that giants should be born even from these. For all of their children were not giants; but there were more then than in the remaining periods since the deluge. And it pleased the Creator to produce them, that it might thus be demonstrated that neither beauty, nor yet size and strength, are of much moment to the wise man, whose blessedness lies in spiritual and immortal blessings, in far better and more enduring gifts, in the good things that are the peculiar property of the good, and are not shared by good and bad alike. It is this which [Baruch] confirms when he says, "These were the giants, famous from the beginning, that were of so great stature, and so expert in war. Those did not the Lord choose, neither gave He the way of knowledge unto them; but they were destroyed because they had no wisdom, and perished through their own foolishness."