Origin of the idea that "demons" and "fallen angels" are the same thing?

In Derek Prince’s book “They Shall Expel Demons”, in chapter 11, he says demons are “disembodied spirit beings which have an intense craving to occupy physical bodies”, preferably human. And that their origin is unclear and there have been two main theories that proponents claim are supported by scripture:

1. Fallen Angels associated with Satan in his rebellion against God.
2. Disembodied spirits from a pre-Adamic race, which race perished under some judgment of God not recorded in detail in the Bible. And “I do not believe that scripture provides us with sufficient evidence to say with certainty which, if either, of these theories, is correct...”

Paraphrasing and quoting him in chap 11:
I must say however that on the basis of my experience and the Bible, I find it hard to believe the demons are fallen angels. It is not scriptural to say demons operate continually on the plane of earth (Eph 6:12 fallen angels are still in the heavenly places, but just not the third one w God 2Cor 12:2-4. And in his experience some of them are weak, cowardly, even ridiculous, which angels, even fallen, wouldn't be.

—- (Chapter two discusses terminology. Diamonia and diablos unfortunately both translated to devil in KJV even though Diablos just means slanderer and is used to refer to Satan in the greek. “There are many demons but only one devil” He says corrected in later English translations. He says the bible says unclean spirits, evil spirits, and demons (both diamonia and diamoniana(?)) are all three diff terms for the same thing and yet they ARE part of Satan’s evil kingdom.)

"'Every kingdom divided against itself goes to ruin, and a divided household falls. Equally if Satan is divided against himself, how can his kingdom stand?--since as you would have it, I drive out the devils by Beelzebub. If it is by Beelzebub that I cast out devils, by whom do your own people drive them out? If this is your argument, they themselves will refute you. But if it is by the finger of God that I drive out the devils, then be sure the kingdom of God has already come upon you'" (Luke 11:17-20 NEBNT).

Although only six versions of the English Bible use the words devil or devils instead of demon and demons, let us assume for the sake of argument that the translation teams for these six versions (viz., KJV, KJVA, TMB, TYN, WYC, and NEBNT) had good, sound reasons for using devil(s) instead of demon(s). To dismiss offhand their scholarly efforts on the basis of the similarity between the transliteration of the Greek words daimôn and daimonion and our English words demon and demons and the difference between the transliterated Greek word for devil, diabolos, is not wise.

Assuming, then, that the NEB's translation of "devils" is a legitimate--albeit minority--one, I suggest we look at another key passage which links "the devil" [i.e., Satan/Beelzebub] with "his angels" this time, and not with his "demons":

"Then shall he [viz., the King] say also unto them on the left hand, 'Depart from me, ye cursed, into the eternal fire which is prepared for the devil and his angels'" (Matthew 25:41 ASV).

Laying aside for a moment the translational difficulties associated with your question (and translational issues are not to be treated lightly, I might add) and using simple logic, the answer to your question about whether or not demons are also fallen angels would seem to involve the following premises:

• Jesus agreed with his critics--the Pharisees--in Luke 11 that there is in fact a bifurcation (or division of labor) between Beelzebub, the prince of demons, and his the demons under his command (or within his principality, so to speak).

• There are many different denotations attached to the Greek words for demon, demons, devil, and devils. A surprising use of the word devil by our Savior himself is found in John 6:70 NAS: "Jesus answered them, 'Did I Myself not choose you, the twelve, and yet one of you is a devil?'"

  Jesus was not calling Judas Iscariot "the devil" (i.e., Satan), but he was linking Judas's behavior to the influence of the devil. Judas was, in a sense, the weed (an unbeliever) which sprang up with the wheat (the true believers), and in a way which is not easily explained, Jesus chose all "the Twelve," including Judas, knowing full well in advance that Judas's heart was not right before God.

• As one web site observes, "Ancient Greek writers often used the word daimôn in the meaning 'god,' 'goddess,' 'the gods,' 'deity,' and 'divine power.' Many times, they used it in a [way which is] similar [to the] way . . . [they used] the word theos."

• If Satan is both a fallen angel (at least according to the majority of conservative and Evangelical theologians and biblical scholars) and is also the prince demons (or devils); and if the king in Jesus' parable in Matthew 25 identifies angels with those spirit-beings who are under the command of their prince, Beelzebub; then we are not stretching too much to suggest that in the world system, or cosmos (see 1 John 2:15 ff.) of "the evil one," his demons or devils are "his angels."

• And finally, if Satan has hordes of angels at his disposal to help him in his dastardly deeds, since he is not omnipresent, omniscient, or omnipotent, then we can conclude with a good degree of certainty that they are not God's angels, but they are Satan's angels and they, like their prince, are also fallen.

Origin of the idea that “demons” and “fallen angels” are the same thing?

There is a description of angels that went bad.

Jude 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.

While Revelation is often called allegorical or metaphorical, a case can be made that a third of the angels went with Satan when he rebelled.

Revelation 12:3-4 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.

Satan is called a former angel.

Ezekiel 28:14-16 Thou art an anointed cherub who is covering, And I have set thee in the holy mount, God thou hast been, In the midst of stones of fire thou hast walked up and down. Perfect art thou in thy ways, From the day of thy being produced, Till perversity hath been found in thee. By the abundance of thy merchandise They have filled thy midst with violence, And thou dost sin, And I thrust thee from the mount of God, And I destroy thee, O covering cherub, From the midst of the stones of fire.

Isaiah 14:12-15 How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the most High. Yet thou shalt be brought down to hell, to the sides of the pit.

A hierarchy is referred to in Satan's realm which would include the fallen angels at the lower levels.

Luke 11:18 If Satan also be divided against himself, how shall his kingdom stand? because ye say that I cast out devils through Beelzebub.

Ephesians 6:12 For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.

We see that judgment is given those angels that are not faithful.

2 Peter 2:4 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;

The Book of Watchers, in the aprocryphal 1 Enoch*, is the earliest specific, pre-Christian reference to the fallen angels, at least in the Septuagint version.

Annette Yoshiko Reed says in an innovative twist on earlier Jewish traditions, Justin Martyr (c. 100-165) proposed that the progeny of the fallen angels, described in Genesis 6.1–4, became demons who masquerade as deities and trick pagans to explain the origins of pagan worship (cf Psalm 96:5: "For all the gods of the nations are idols..." ).

Jeffrey B. Russell says, in The Prince of Darkness, page 63, that after the fifth century, the fallen angels and demons were united in one and the same category of being. Satan, a fallen angel himself, was only partly absorbed into that category.

So we can say the idea that demons and angels are the same thing came from The Book of Watchers, inspired by Genesis 6:1-4, was developed by Justin Martyr, and finally became Christian doctrine in the fifth century.

*1 Enoch

Chapter 6:

1 And it came to pass when the children of men had multiplied that in those days were born unto 2 them beautiful and comely daughters. And the angels, the children of the heaven, saw and lusted after them, and said to one another: 'Come, let us choose us wives from among the children of men 3 and beget us children'...

Chapter 7:

1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms 2 and enchantments, and the cutting of roots, and made them acquainted with plants. And they 3 became pregnant, and they bare great giants, whose height was three thousand ells: Who consumed 4 all the acquisitions of men. And when men could no longer sustain them, the giants turned against 5 them and devoured mankind. And they began to sin against birds, and beasts, and reptiles, and 6 fish, and to devour one another's flesh, and drink the blood. Then the earth laid accusation against the lawless ones.

A bit late here, but just wanted to add that the concept that fallen angels are demons (leaving beside the evidence in the New testament, which is not altogether satisfactory), is far older than Justin Martyr developments. The Slavonic Book of Enoch, usually believed to have been written in the 1st century CE, is very explicit about it. First we are told that Satan and his angels rebelled and were thrown from heaven:

“But one from the order of the archangels deviated, together with the division that was under his authority. He thought up the impossible idea,that he might place his throne higher than the clouds which are above the earth, and that he might become equal to my power. And I hurled him out from the height, together with his angels. And he was flying around in the air, ceaselessly, above the Bottomless.” – 2 Enoch 29:4-5

And later we are told that he turned into a demon:

“And the devil understood how I wished to create another world, so that everything could be subjected to Adam on the earth, to rule and reign over it. The devil is of the lowest places. And he will become a demon, because he fled from heaven; Sotona, because his name was Satanail. In this way he became different from the angels. His nature did not change, (but) his thought did, since his consciousness of righteous and sinful things changed. And he became aware of his condemnation and of the sin which he sinned previously." – 2 Enoch 31:3-7

The Slavonic Book of Enoch is a Jewish pseudepigrapha, but it's assumed among scholars that it has important Christian interpolations. These passages are probably of Christian origin, given that the story of the fall in Jewish tradition was that of the Watchers, and demons were the spirits of dead Nephilim.

Another pseudoepigraphical work with important Christian interpolations is the Testament of Solomon (between 1st and 5th century CE). Here the demon Beelzebub is imprisoned by Solomon together with other demons, and there's this exchange:

"Why art thou alone, prince of the demons?" And he said to me: "Because I alone am left of the angels of heaven that came down. For I was first angel in the first heaven being entitled Beelzeboul. And now I control all those who are bound in Tartarus."

As close as I can understand these Scriptures they apply to the fallen Angels:

Revelation 12:7 through 9 NKJV And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, 8 but they did not prevail, nor was a place found for them in heaven any longer. 9 So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him.

That does not say that those Angels are Demons, However; other Scriptures do lead us to understand they are:

Luke 11:18 NKJV If Satan also is divided against himself, how will his kingdom stand? Because you say I cast out demons by Beelzebub.

The Scriptures refer to Satan's helpers as both angels and demons:

Matthew 25:41 NKJV "Then He will also say to those on the left hand, 'Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels:

compared to:

Revelation 16:13 and 14 NKJV And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet. 14 For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.

Even though I cannot give you any verses which say that Demons are fallen Angels these Scriptures taken together along with the description of the Angel opening the gate to the bottomless pit leaves little doubt in my mind that they are the same.

Hope this helps and sorry to not give you something more solid to use.