Demons are not fallen angels nor are fallen angels demons.
Fallen angels are much greater in power than demons. Nowhere are we told to cast out fallen angels; however, we are told in Mark 16 to cast out demons:
And these signs shall follow them that believe; In My name shall they cast out devils; they shall speak with new tongues … (Mark 16:17).
Yet Michael the archangel, when contending with the devil, he disputed about the body of Moses, durst not bring against him a railing accusation, but said, “The Lord rebuke thee (Jude 1:9).”
Demons can be cast out of a person. Fallen angels don’t inhibit a person, and second, have no power over them in the sense of the demonic. The Bible never gives Christians the authority to “rebuke” a fallen angel. Zechariah 3:2 tells us that it is the Lord who rebukes fallen angels. Even Michael, one of the most powerful of the angels, did not dare to accuse Satan, but rather said, “The Lord rebuke you” (Jude 1:9). In Daniel, the Archangel Michael is sent to fight the fallen angel behind the King of Persia.
Fallen Angels are said to be a concept developed in Jewish mythology from an interpretation of the Book of Enoch. However, the concept of fallen angels is mainly a Christian concept, not a Jewish one.
First, the Bible and ancient texts agree as to the fact that some angels are fallen or rejected by God. A good example of this rejection can be seen in several sections of the Book of Revelation, 2 Peter, and the Book of Enoch.
And the fifth angel sounded, and I saw a star fall from heaven unto the earth (an angel): and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit. And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power. And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads. And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man. And in those days shall men seek death, and shall not find it; and shall desire to die, and death shall flee from them. And the shapes of the locusts were like unto horses prepared unto battle; and on their heads were as it were crowns like gold, and their faces were as the faces of men. And they had hair as the hair of women, and their teeth were as the teeth of lions. And they had breastplates, as it were breastplates of iron; and the sound of their wings was as the sound of chariots of many horses running to battle. And they had tails like unto scorpions, and there were stings in their tails: and their power was to hurt men five months. And they had a king over them, which is the angel of the bottomless pit, whose name in the Hebrew tongue is Abaddon, but in the Greek tongue hath his name Apollyon (Revelation 9:1-11).
You might notice in these verses an angel is called, “the angel of the bottomless pit.” This certainly would not be a godly angel from the description given in these verses. The most common verse used to explain fallen angels is in Revelation 12:7-9:
And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, and prevailed not; neither was their place found any more in heaven. And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
Peter underscores that there were angels that “rebelled, “sinned” and were “cast out.”
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 … (2 Peter 2:4, KJV).
Fallen Angels Further Described in Genesis
In Genesis, the fallen angels are called, “the sons of god” (Genesis 6:4). Genesis is a much older writing than the Book of Enoch. The dating of the Book of Enoch is around the 2nd-1st Century BC. The writings of the Book of Genesis are dated to at least that of the United Monarch, somewhere around 1200 BC. In Genesis, fallen angels are known as the “bene ha-’elohim” (“sons of god”) who have sex with human women. The sons of God saw the daughters of men that they were fair; and they took them wives of all that they chose … The Nephilim were in the earth in those days, and also after that, when the sons of God came in unto the daughters of men (Genesis 6:4). This union of fallen angels created giants.
These giants were seen before the Flood, and again after the Flood when the children of Israel attempt to enter Canaan. A fallen angel would have had to have been the father of Goliath since Genesis 6:4 says this is how they are created. This account gives rise to, and an understanding of, the giant races of antiquity by the union of demigods with human wives, as portrayed by Greek Mythology.
The term “bene ha-’elohim,” that is, “sons of God,” used in Genesis 6:2-4 and Job 1:6 and 2:1, is applied to angels. In Job, the “sons of God” are represented as beings appearing before the Throne of God in heaven. Most scholars now interpret “sons of God,” “bene ha-’elohim,” as referring to supernatural beings in accordance with the meaning of the expression in the other passages.
The intention of the original writer of Genesis was to account for the rise of the giants of antiquity by the union of angels with human wives. This interpretation accords with Enoch chapters 6-7, and with Jude 1:6, where the unnatural sin of the men of Sodom who went after “strange flesh” is compared with that of the angels (compare 2 Peter 2:4).