Exactly what that means, may be debatable; obviously the form of delivery is poetic. But why suppose it means only less than what it says?
Jesus was clearly thinking in terms of some kind of supernatural rebellion affecting the humans of earth and being affected in turn, not only by His ministry and actions, but by the ministry of His authoritative representatives.
It might be replied that He was only using language in a way that was accessible to the beliefs of the people of that time–and I have no problem believing that some of that happened. (In principle, His use of parabolic teaching amounts to the same thing, for example. Although, interestingly, it is not in His overtly parabolic teaching that the demonology of the Gospels is usually found; rarely so, rather.)
But, even if this is true, that is a much different kind of claim than to say that the concept of fallen supernatural entites with one or more strong leaders, whose fall is somehow supercessive to our own rebellion as humans, is “not even vaguely referred to in the New Testament”.
This is why I am asking FB to be more clear about what kind of “myth” he is saying does not even appear before the 2nd century, including without even the vaguest reference in the NT texts. I might or might not agree with him–the complex demonologies of Gnostic systems, for example, and some post-Gnostic Christian writings, do have very little parallel in any NT text (although on the other hand they do have some real parallel in post-OT Jewish religious writing and traditions, which is quite fervid, including in pre-Christian material. ) The same is true about “revelatory” texts (Jewish, Christian and otherwise) describing hell in Dantesque detail; there is no parallel to this in NT texts either.
But I do not consider these to be the basic “myth” (in the sense of a meaningful story, aside from the question of accurate metaphysical truth or history) of rebel supernatural entities under God–a basic myth that I do find well represented in OT and NT texts, if not quite in any obviously systematic fashion. (Which should not be surprising, since systematic theology of any kind is rarely if ever a topic in any of the canonical texts.)
Broadly speaking, this myth found in the scriptures is that all sentient creatures were originally created good, but that just as human souls have rebelled and fallen away from God so have more powerful spirits who were once in authority over the natural system (and who still retain some significant authority and power over the system, even in their rebellion).
After that, details become more spotty, but again broadly speaking the following themes (presented in no particular order) can be adduced:
1.) God punished some of these spirits by incorporating them into physical material of various sorts; and/or by discorporating them into an unseen prison–in either case their ability to interfere with natural history was reduced but not entirely eliminated. (Often the sea is used in reference to one or even both of those concepts: as the actual prison of primordial rebels against God, or as representing the spiritual prison of the unseen gloom. The air itself is also sometimes spoken of as their abode or prison.)
2.) Intelligent mighty reptiles are often thematically connected with the most powerful of these rebel spirits.
3.) God’s loyal angels are still warring, along with God in various ways (directly and indirectly), against these rebel spirits.
4.) The rebel spirits have a special hatred for women and children; seeking the corruption and/or enslavement of the former, and the murder of both. Men are typically induced by the rebels to fulfill these goals. Paradoxically, the corruption of women is often aimed at increasing the rebels’ power over men, precisely for the purpose of accomplishing this special hatred against women and children.
5.) The rebels (with some indication that this is due to an original setup of authority, by God, which they have abused) are busily engaged in overtaking earthly cultures, and have largely succeeded in doing so. (Although those cultures also have guardian angels assigned to them who are warring against the usurpers. And not very successfully, either! But still, things could be, and have been, and will be much worse.)
6.) The mythologies of cultures surrounding Israel, as a result, do in fact contain a lot of religious truth; but in a fashion that has been perverted and distorted by the rebel angels. Consequently, it is not trustworthy and ought to be avoided as a seductive poison. (Except where corrected and incorporated into the Jewish canon, of course. )
7.) God intends to subdue and repatriate these entities, even the greatest cosmic rebels, leading them back to a loyal covenant with Him. This won’t be finished anytime soon, but God is already going about this process, and has been doing so for a very long time.
8.) Also, there may be “neutrals knocking about” (to put it in Lewis’ terms)–but due to human corruption it is (generally speaking) not safe for us to be dealing with them at this time. Sooner or later they’ll be taking sides, just as all rational creatures are expected to do.
9.) Humans were expected to work with God in governing (and perhaps in recovering an already fallen) Nature, but as a species we have been corrupted into cooperating with the plans of the rebels more often than with God. (In effect, we ourselves may be said to have been one of those “neutrals knocking about” at one time, although not anymore.)
10.) Part of God’s plan seems to be to teach the rebels better by actually letting them have their way (to some far extent), despite the tragedy this results in for the short or medium term. He even allows them occasional freedom from their imprisonment while still being rebels, for this purpose. (Among other purposes, this also ends up showing us lesser beings that teaming up with the demons is a bad idea and only brings tragedy to us from the demons themselves, who only want to use and abuse us for their own selfish gratifications. When we insist on such a team-up anyway, God occasionally grants it. More pity us.)
11.) The self-sacrifice of God through Jesus (somehow, setting aside particular Christological understandings), on the cross, is somehow the single most crucial deed God has ever done or will ever do, toward reconciling Himself with those in the heavens or on the earth or under the earth (rebels being the only creatures who need “reconciliation” with Him, of course). Relatedly, much of Jesus’ earthly ministry was aimed toward undermining, and in some incidental cases even directly challenging, the entrenchment of these rebels among humanity and the natural order.
12.) Some of these entities are very much stronger than the others, and (naturally) seek and achieve power over the others. At any given time we can expect there to be one strongest rebel.
13.) There is some indication that the strongest and most authoritative rebel, over the others, is also the one among them who first and originally fell.
14.) Despite their rebel status, God’s authority over them remains; and without them necessarily realizing how, they still end up serving His purposes as servants.
15.) These rebels seek to become like God (which they’re necessarily going to fail at); and in competition against Him (as well as for their own selfish gratification) they seek to be worshiped and served by lesser creatures (including humans)–which they’ve been very successful at doing.
16.) We can expect this process to continue in recurring waves throughout human history, in different forms and fashions; including “lesser” great tribulations (so to speak), until the final tribulation, which is going to be the worst thing our world has ever gone through. After this final tribulation, though, in the Day of the Lord to come, the rebellion will be finally defanged by the active judgment of God and only reconciliation will remain to be completed.
17.) The leader of the rebels spirits was greatly impressed by the advent of Jesus, in at least some way, and wants to emulate it himself. He will do so before the final tribulation. Until then, there will be (and have been) precursors to this event (just like there were precursory “echos” of the advent of the Son of God), some of which have been described in scripture, where humans in corrupt authority (in conjunction with the rebels, though not necessarily in conscious cooperation with them) have promoted themselves as being “like the Most High”–and proceeded to terribly abuse the ones under their authority.
18.) The fall of the rebel spirits didn’t happen all at once, and is still going on. Some loyalists are on the verge at any given time. (This may also represent the idea of neutrals taking sides throughout history, however.)
Whatever these themes may actually mean, and however they might best be interpreted (theologically, sociologically, anthropologically, psychologically, whatever, or in whatever combination of solution): they are not “not even vaguely” found in the scriptures.