Demons are angels who fell: this is how the official teaching of the Christian Church tells us. Probably, everyone knows the story of the rebellion of the angels - there are hints of it in the Bible, Christian thinkers lead to it, the great literary description of angelomachy is available in J. Milton.
But, nevertheless, I will briefly recall this story: one of the angels named Lucifer ("the light-bearer") became proud of his power and decided to take the place of God. Lucifer raised a mutiny in heaven and led a third of the entire angelic army. Archangel Michael decided to speak out against them, because he believed God without question. As a result: the rebels, along with Lucifer, were thrown from heaven into the underworld and became demons, whose main purpose now is to sow evil.
This story has a large number of interpretations, but now we will give only the original versions of the origin of demons, which are fundamentally different from the orthodox:
1. In the Middle Ages there was a point of view, which said that demons were originally created by God to commit atrocities. Those who defended this theory relied on a quote from the Book of Isaiah, where through the mouth of God it is said: “I create a destroyer for destruction” (54, 16). In the rabbinic treatises, it is said that Satan was created on the sixth day of creation together with Eve;
2. In the teachings of the Bogomil heretics, and in popular beliefs that have not yet got rid of pagan dualism, Satan (Satanael) is not represented by God's creation, but an independent figure that opposes God as the Persian Ahriman. These forces - good and evil - took part in the process of creating the world; in contrast to God's angels, Satan creates his army, striking the flint with his staff.
3. The Apocryphal Book of Enoch tells the story of the cohabitation of angels with human girls. The angels who traded heaven for human pleasure were cursed by God and became demons. This hypothesis in the Middle Ages was supported by a large number of church authorities.
4. In the same Book of Enoch it is said that from the marriage of a fallen angel with an earthly woman, a tribe of terrible giants emerged. And when God destroyed all the giants, evil spirits came out of their bodies.
5. The ancient Jews were sure that most of the evil spirits were born from the intercourse of Adam with female spirits (or Eve with male spirits) during those 130 years into which Adam and Eve were separated after the Fall. A huge number of demons were spawned from Adam by his first wife, Lilith, who later turned into a demon herself.
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6. And finally, according to later beliefs, the army of hell is constantly replenished at the expense of the souls of great sinners; children cursed by their parents, and the offspring of succubi and incubi. But all these are demons of the lowest rank, just like all sorts of vampires, ghosts and werewolves, which also make up Satan's army.