A fallen angel in Abrahamic traditions is an angel that has been exiled or banished from Heaven. Often such banishment is a punishment for disobeying or rebelling against God.

One early source for information on angelology as well as demonology, is the Persian prophet Zoroaster, who is thought to have influenced Judeo-Christian beliefs .

The best-known fallen angel is Satan. According to some traditions, fallen angels will roam the Earth until Judgment Day, when they will be banished to Hell.

There are a number of beliefs regarding fallen angels. Many focus on issues of free will, lust, pride, or the incomprehensibility of the acts of God. An act by God of creation was forseen as resulting in multiple outcomes, with each of these three doctrines that were traits held by certain angels. These fallen angels foresaw the doom impending. In one entity, that is both omnipotent and omnipresent, who gives certain ideas to humanity and expects only one right way to live - by universal morals, ethics and faith. However, given the doctrine of free will, each one of these ideas leads one to speculate on the basis of duality of right and wrong, good and evil, and heaven and hell.

This hypothesis focuses on the concept that God has two opposite sides; a divine (or bright) side, and a shadow (or dark) side. It is believed that if mortals were to see God's bright side, His radiance would incinerate them instantly. Therefore, a dark side was necessary for Him to communicate with mortals. As Judaism evolved, so, too, did the sides of God. Initially, the shadow side became "God's Word" and then the "Voice". In time, it came to be considered a separate being with its own free will, far removed from the bright side of God. The dark side began to take on some negative aspects with which mortals were familiar — destruction, betrayal, temptation, etc. Eventually, the dark side of God became known as Satan. There is no Biblical evidence supporting this hypothesis. The word "Satan" is a version of the Hebrew word "ṣāṭān" or "ShTN", meaning "adversary"

It is generally accepted by most Christians that the fallen angels were cast out of Heaven due to free will. Pride, being the most grave of sins, is usually taught as the reason the angels were expelled from Heaven.

Origen

Origen, a father of the early Christian Church, believed that God had created all angels to be equal and free. However, in possessing the power of free will, some of them began to move further away from God of their own volition.

Origen maintained that those who drifted the least remained in the regions closest to God, while those who moved further out fell into lower air, and became what we know as "angels". This is the lowest order of the angelic hierarchy. Those who moved still farther away became humans, and finally those who moved the farthest away from God became "demons", who reside in Hell.

Origen states metaphorically that, although some angels fell and became humans or demons, all hope is not lost. By practicing virtue, men and demons can again become angels. While considered an early Father of the Church, Origen was deemed a heretic as a result of some of his writings and teachings, which did not conform to accepted scripture or tradition. Mainly, his concept of Apocatastasis, the belief that all beings (humans, fallen angels, demons and Satan) will return to God through God's love and mercy, was deemed unacceptable at that time. His excommunication was posthumously reversed.

The following comes from a series of ancient texts referenced in the Bible called "The Three Books of Enoch", a set of books found in the Pseudepigrapha of the Old Testament.

According to these books, it is because of lust that some angels fell from Heaven. God asked the "Watchers" (Grigori), a select group of angels, to assist the Archangels in the creation of Eden. Those Grigori who descended to Earth saw the daughters of men and became enchanted with them. Consequently, the Grigori began to reveal to man some of the secrets of Heaven, such as astrology, weapons production, and the vanity of enhancing the face and body with perfumes and cosmetics. The Grigori then fell in love with human women. According to the text, some of the Grigori even took wives and created offspring, giants known as the Nephilim. This made God so angry that he cursed those Grigori who had betrayed Him, threw them out of Heaven, made them mortal and transformed them into demons. God sent the Great Flood to cleanse the Earth of the wanton killing and destruction perpetrated by the Nephilim. Notable angels who fell in this account are Semyazza, Samael, and Azazel. A single verse from the Book of Genesis in the Old Testament alludes to the "sons of God" intermarrying with the "daughters of Man" to create a species known as "Nephilim".