Whatever may be said of this theory of the
Rabbis, that the air is full of demons, and that men are in
danger of receiving them into their systems it may certainly be
said that in the days of the early Christians the air was
dangerously full of demonologies, and that men were in peculiar
peril of adopting erroneous doctrines on this matter.
It must be remembered, on the one hand, that many of the Gospel miracles, and particularly the casting out of devils, must in any case have given the faithful a vivid sense of the existence and power of the evil spirits. At the same time, as we have seen, Scripture itself did not furnish any full and clear information in regard to the origin and the nature of these powerful enemies; on the other hand, it may be observed that the first Christian converts and the first Christian teachers were for the most part either Jews or Greeks, and many of them were living in the midst of those who professed some or other of the old Oriental religions. Thus, while they naturally wished to know something about these matters, they had but little definite knowledge of the truth, and on the other hand their ears were daily filled with false and misleading information. In these circumstances it is scarcely surprising to find that some of the earliest ecclesiastical writers, as St. Justin, Origen, and Tertullian, are not very happy in their treatment of this topic.
There was, moreover, one fruitful source of error which is rather apt to be forgotten. Now that common consent of Catholic commentators has furnished a better interpretation of Genesis, vi, 2, and conciliar definitions and theological arguments have established the fact that the angels are purely spiritual beings, it may seem strange that some early Christian teachers should have supposed that the phrase, sons of God, could possibly mean the angels or that these pure spirits could have taken unto themselves wives of the daughters of men. But it must be borne in mind that the old commentators, who read the Septuagint or some derivative version, did not put this interpretation on the passage; the word itself was in the text before them, that is to say, the old Greek Bible expressly said that "the Angels of God took wives of the daughters of men". This unfortunate reading was certainly enough to give a wrong direction to much of the demonology of early Christian writers and those who went astray in other matters also naturally adopted peculiar ideas on this subject.
In some ways one of the most remarkable examples of this mistaken demonology is that to be found in the pseudo-Clementine Homilies (Hom. viii, ix). The writer gives a very full account of the mysterious episode of Genesis, vi, 2, which, in common with so many others, he takes to be the origin of the demons who were in his view, the offspring of the supposed union of the angels of God and the daughters of men. But on one point, at any rate, he improves the story and does something to lighten our initial difficulty.
The first objection to the legend was, that the angels as pure spirits, were plainly incapable of feeling sensual passions; and it was possibly a keen sense of this difficulty that led some who had adopted the story to deny the spirituality of the angelic nature. But the moralist evades it in a more ingenious manner. According to his account, the angels were not overpowered with the passion of sensual love while they were as yet in their purely spiritual state; but when they looked down and witnessed the wickedness and ingratitude of men whose sins were defiling the fair creation of God, they asked of their Creator that they might be endowed with bodies like those of men, so that coming down to earth, they might set things right and lead a righteous life in the visible creation.
Their wish was granted, they were clothed in bodies and came down to dwell on earth. But now they found that with their raiment of mortal flesh they had acquired also the weakness and passions which had wrought such havoc in men, and they too, like the sons of men, became enamoured of the beauty of women and, forgetting the noble purpose of their descent to earth, gave themselves up to the gratification of their lust, and so rushed headlong to their ruin. The offspring of their union with the daughters of men were the giants -- the mighty men of superhuman build and superhuman powers, as became the sons of incarnate angels, yet at the same time mortal, like their mortal mothers. And when these giants perished in the Flood their disembodied souls wandered through the world as the race of demons.
Assyrian and Akkadian Demonology
Early Christian Demonology
Medieval to Modern Demonology
The Book of Enoch