The Devil (Satan) Is a Fallen Angel
The devil is one of the multitude of angels created by God, but who afterwards did not wish to serve God, that is, fell away from God, fell:
“You were the seal of perfection, full of wisdom and perfect in beauty. You were in Eden, the garden of God; every precious stone was your covering: the sardius, topaz, and diamond, beryl, onyx, and jasper, sapphire, turquoise, and emerald with gold. The workmanship of your timbrels and pipes was prepared for you on the day you were created. You were the anointed cherub who covers; I established you; you were on the holy mountain of God; you walked back and forth in the midst of fiery stones. You were perfect in your ways from the day you were created, till iniquity was found in you. By the abundance of your trading you became filled with violence within, and you sinned; therefore I cast you as a profane thing out of the mountain of God; and I destroyed you, O covering cherub, from the midst of the fiery stones. Your heart was lifted up because of your beauty; you corrupted your wisdom for the sake of your splendor; I cast you to the ground, I laid you before kings, that they might gaze at you. You defiled your sanctuaries by the multitude of your iniquities, by the iniquity of your trading; therefore I brought fire from your midst; it devoured you, and I turned you to ashes upon the earth in the sight of all who saw you. All who knew you among the peoples are astonished at you; you have become a horror, and shall be no more forever” (Ezek. 28:12-19)
Such angels who have fallen away from God are called by various words: demon (Matt. 8:31; Tob. 6:8), messenger (angel) of Satan (2 Cor. 12:7), unclean spirit (Matt. 10:1), devil (in popular speech). Satan is also called the serpent (Gen. 3:1; Is. 27:1; Rev. 12:9; 20:2), the dragon (Rev. 12:9; 20:2), Lucifer, son of the morning (Is. 14:12), Leviathan (Is. 27:1), the ruler of this world (John. 12:31), the wicked one (Matt. 13:19), a lying spirit (1 Kin. 22:22). The devil is not an impersonal feeling, not an abstract impulse in man, not an allegory. He is a real being, a person. The devil is the first being that sinned:
“He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil” (1 John. 3:8)
The Devil Has His Own Will
“and that they may come to their senses and escape the snare of the devil, having been taken captive by him to do his will” (2 Tim. 2:26)
The Devil Acts by Suggesting Evil Thoughts to a Person
“But Peter said, ‘Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back part of the price of the land for yourself?’” (Act. 5:3)
“And supper being ended, the devil having already put it into the heart of Judas Iscariot, Simon’s son, to betray Him” (John. 13:2)
Satan Fell from Heaven
“I saw Satan fall like lightning from heaven” (Luke. 10:18; Ezek. 28:12-19)
He Drew Other Angels After Him
“For if God did not spare the angels who sinned, but cast them down to hell and delivered them into chains of darkness, to be reserved for judgment” (2 Pet. 2:4)
Because of the Devil, Death Came into the World
“but through the devil’s envy death entered the world, and those who belong to his company experience it” (Wis. 2:24)
The Devil Is the Adversary of People. He Seeks Whom to Draw After Him into Perdition
“Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour” (1 Pet. 5:8)
The Devil Is the Adversary of Human Salvation
“Now the parable is this: The seed is the word of God. Those by the wayside are the ones who hear; then the devil comes and takes away the word out of their hearts, lest they should believe and be saved” (Luke. 8:11-12)
“Then he showed me Joshua the high priest standing before the Angel of the LORD, and Satan standing at his right hand to oppose him” (Zech. 3:1)
The Devil Disguises Himself as an Angel of God
“And no wonder! For Satan himself transforms himself into an angel of light” (2 Cor. 11:14)
The Devil Flees from Those Who Submit to God and Turn Away from the Devil
“Therefore submit to God. Resist the devil and he will flee from you” (James. 4:7)
The Devil Knows Holy Scripture and Tempts with It, Distorting Its Meaning
“and said to Him, ‘If You are the Son of God, throw Yourself down from here. For it is written: “He shall give His angels charge over you, to keep you,” and, “In their hands they shall bear you up, lest you dash your foot against a stone.”’ And Jesus answered and said to him, ‘It has been said, “You shall not tempt the LORD your God”’” (Luke. 4:9-12)
The Devil Is the Father of Lies
“He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it” (John. 8:44)
The Devil Is Able to Work Wonders, to Bring Down Fire from Heaven
“And the LORD said to Satan, ‘Behold, all that he has is in your power; only do not lay a hand on his person.’ So Satan went out from the presence of the LORD. <...> another also came and said, ‘The fire of God fell from heaven and burned up the sheep and the servants, and consumed them; and I alone have escaped to tell you!’” (Job. 1:12,16)
“The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders” (2 Thess. 2:9)
The Devil Disputes with the Angels of God
“Yet Michael the archangel, in contending with the devil, when he disputed about the body of Moses, dared not bring against him a reviling accusation, but said, ‘The Lord rebuke you!’” (Jude. 1:9)
A Place of Eternal Torment Has Been Prepared for the Fallen Angels
“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’” (Matt. 25:41).
“The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever” (Rev. 20:10)
The Demons Know This
“And suddenly they cried out, saying, ‘What have we to do with You, Jesus, You Son of God? Have You come here to torment us before the time?’” (Matt. 8:29)
The Demons Do Not Seek God’s Will Freely (Voluntarily), but Cannot Disobey God’s Command
“Then they were all amazed, so that they questioned among themselves, saying, ‘What is this? What new doctrine is this? For with authority He commands even the unclean spirits, and they obey Him’” (Mark. 1:27)
“So the demons begged Him, saying, ‘If You cast us out, permit us to go away into the herd of swine’” (Matt. 8:31)