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Christian demonology is the study of demons from a Christian point of view. It is primarily based on the Bible (Old and New Testaments), the exegesis of these scriptures, the scriptures of early Christian philosophers and hermits, tradition and legends incorporated from other beliefs.
In monotheistic religions, the deities of other religions are sometimes interpreted or created as demons. The evolution of the Christian Devil and pentagram are examples of early rituals and images that showcase evil qualities, as seen by the Christian churches.
Since Early Christianity, demonology has developed from a simple acceptance of demons to a complex study that has grown from the original ideas taken from Jewish demonology and Christian scriptures. Christian demonology is studied in depth within the Roman Catholic Church, although many other Christian churches affirm and discuss the existence of demons.
According to the Book of Enoch (which is currently only canonical in the Eritrean and Ethiopian Orthodox Churches but was referred to by the early Church fathers), the disembodied spirits of the Nephilim are demons. Enoch explains;
"And now, the giants, who are produced from the spirits (Angels) and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling. Evil spirits have proceeded from their bodies; because they are born from men and from the holy Watchers is their beginning and primal origin; they shall be evil spirits on earth, and evil spirits shall they be called. [As for the spirits of heaven, in heaven shall be their dwelling, but as for the spirits of the earth which were born upon the earth, on the earth shall be their dwelling.] And the spirits of the giants afflict, oppress, destroy, attack, do battle, and work destruction on the earth, and cause trouble: they take no food, but nevertheless hunger and thirst, and cause offences. And these spirits shall rise up against the children of men and against the women, because they have proceeded from them. From the days of the slaughter and destruction and death of the giants, from the souls of whose flesh the spirits, having gone forth, shall destroy without incurring judgement". (I Enoch 15:8-12, 16:1 R.H. Charles)
There are many demons in Christian demonology, many of which were added because some Christian theologians concluded that all pagan deities were demons.
In 1467, Alfonso de Spina asserted that the number of demons was 133,316,666. This idea that one third of the angels turned into demons seems to be due to an exegesis of the Book of Revelation 12:3-9.
Johann Weyer, in his Pseudomonarchia Daemonum (1583), after a complicated system of hierarchies and calculations, estimated the number of demons as 44,435,622, divided into 666 legions, each legion composed by 6,666 demons, and all of them ruled by 66 hellish dukes, princes, kings, etc. The Lesser Key of Solomon (17th century) copied the division in legions from Pseudomonarchia Daemonum but added more demons, and so more legions. It is suggestive that both Spina and Weyer used 666 and other numbers composed by more than one 6 to calculate the number of demons (133,316,666 demons, 666 legions, 6,666 demons in each legion, 66 rulers).
Gregory of Nyssa, in the 4th century, believed in the existence of male and female demons and supported the idea that demons procreated with other demons and with human women. Other scholars supported the idea that they could not procreate and that the number of demons was constant.
In Christian tradition, demons are evil angels (Revelation 12:7-9), and have the same characteristics as their good angel counterparts: spiritual, immutable and immortal. Demons are not omniscient, but each one has a specific knowledge (sometimes on more than one subject). Their power is limited to that which God allows, so they are not omnipotent. No reference has been made about omnipresence, so it is as yet unclear if they can be in different places at the same time, but according to the tradition of the medieval witches' Sabbath, two conclusions can be reached: either the Devil can be in different places at the same time, or he sends an emissary in his name.
Christian demonology states that the mission of the demons is to induce humans to sin, often by testing their faith in God. Christian tradition holds that temptations come from three sources: the world, the flesh, and the devil.
It is also believed that demons torment people during their life or through possession (Matthew 17:15-16), or simply by showing themselves before persons to frighten them, or by provoking visions that could induce people to sin or to be afraid.
Demons are also believed to try to tempt people into abandoning the faith, commit heresy or apostasy, remain or turn themselves Pagan or venerate "idols" (the Christian term for cult images), and gain the highest number of "Satans" or adversaries of God. (Ephesians 6:12)
In the Book of Luke, it is stated that demons walk "arid places", and finding no rest return to their previous home.
24 “When an impure spirit comes out of a person, it goes through arid places seeking rest and does not find it. Then it says, ‘I will return to the house I left.’ 25 When it arrives, it finds the house swept clean and put in order. 26 Then it goes and takes seven other spirits more wicked than itself, and they go in and live there. And the final condition of that person is worse than the first.” (Luke 11:24-26)(NIV)
Demons can take any desired appearance, even that of an "angel of light" (2 Corinthians 11:14).
"13. For such men are false apostles, deceitful workmen, masquerading as apostles of Christ. 14. And no wonder, for Satan himself masquerades as an angel of light. 15. It is not surprising, then, if his servants masquerade as servants of righteousness. Their end will be what their actions deserve."
(2 Corinthians 11:13-15)
Nevertheless, they were generally described as ugly and monstrous beings by Christian demonologists. Many of these descriptions have inspired famous painters like Luca Signorelli, Hieronymus Bosch, Goya, the artist that made the drawings for the Dictionnaire Infernal, and others.
The idea that demons have horns seems to have been taken from the Book of Revelation chapter 13. The book of Revelation seems to have inspired many depictions of demons.[original research?] This idea has also been associated with the depiction of certain ancient gods like Moloch and the shedu, etc., which were portrayed as bulls, as men with the head of a bull, or wearing bull horns as a crown.
Concerning the weight of the demons, since the 17th century, people have affirmed that they were heavier than common humans.
About the color of the demons' skin, since early times it was associated with black, thinking that they assumed the appearance of a black man, although not all descriptions agreed, giving demons very different aspects. Satan and other demons were also often depicted as black-dressed men, often riding a black horse. When demons appeared in the shape of animals, often they were black. Poets such as Geoffrey Chaucer associated the color green with the Devil, although in modern times the color is red.
Henry Boguet and some English demonologists of the same epoch asserted that witches and warlocks confessed (under torture) that demons' bodies were icy. During the 17th century, this belief prevailed.
Demonic supernatural powers are believed to include psychokinesis, levitation, divination, possession, ESP, telepathy, witchcraft and curses, as well as superhuman strength, controlling the classical elements, animal control and provocation. Demons use variants and combinations of these powers to harass, demoralize, confuse, and disorient the victim, or the willing subject of demonic interest.
Demons are believed to have the power to physically or mentally hurt people, but only within the boundaries of what God will allow. Demons can destroy any material on the earth; these supernatural powers are always inferior to the power of God. God may use His will to cancel or destroy any effect the demon chooses to invoke. According to the gospels, Jesus also had full power over demons, and they always obeyed his commands when he told them what to do, as opposed to a more general sense in which demons only do that which God tells them to. Often Demons are said to create negative emotions, wreaking havoc, ensuing chaos, and disrupting peace.
The incarnation of the demons has been a problem to Christian demonology and theology since early times. A very early form of incarnation of demons was the idea of demonic possession, trying to explain that a demon entered the body of a person with some purpose or simply to punish that one for some allegedly committed sin. But this soon acquired greater proportions, trying to explain how demons could seduce people to have sexual relationships with them or induce them to commit other sins. To Christian scholars, demons didn't always have to manifest themselves in a visible and possible tangible form. Sometimes it was through possession.
New Testament via possession (analogous to invocation)
There are some Biblical mentions of the incarnation of demons, similar in result to possession as in invocation, in the New Testament, according to the Gospels of Matthew, Mark and Luke as they could be seen and heard, as well as banished.
Matthew 8:16 – When the evening had come, they brought unto him many that were possessed with devils: and he cast out the spirits with [his] word, and healed all that were sick:
Mark 1:23-27 – And there was in their synagogue a man with an unclean spirit; and he cried out, Saying, Let [us] alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art, the Holy One of God. And Jesus rebuked him, saying, Hold thy peace, and come out of him. And when the unclean spirit had torn him, and cried with a loud voice, he came out of him. And they were all amazed, insomuch that they questioned among themselves, saying, What thing is this? what new doctrine [is] this? for with authority commandeth he even the unclean spirits, and they do obey him.
Matthew 8:28-33 – And when he Jesus was come to the other side into the country of the Gergesenes, there met him two possessed with demons, coming out of the tombs, exceeding fierce, so that no man might pass by that way. And, behold, they cried out, saying, What have we to do with thee, Jesus, thou Son of God? art thou come hither to torment us before the time? And there was a good way off from them a herd of many swine feeding. So the demons besought him, saying, If thou cast us out, suffer us to go away into the herd of swine. And he said unto them, Go. And when they were come out, they went into the herd of swine: and, behold, the whole herd of swine ran violently down a steep place into the sea, and perished in the waters. And they that kept them fled, and went their ways into the city, and told every thing, and what was befallen to the possessed of the devils.
Other sources via incarnation (analogous to evocation)
Basil of Caesarea also who wrote on this subject. He believed that demons, to materialize, had to condense vapors and with them form the body of a person or animal, then entering that body as if it were a puppet to which they gave life. Henry More supported this idea, saying that their bodies were cold due to the solidification of water vapor to form them (see below). Many authors believed that demons could assume the shape of an animal.
Raoul Glaber, a monk of Saint-Léger, Belgium, seems to have been the first in writing about the visit of a demon of horrible aspect in his Historiarum suis temporis, Libri quinque (History of his time, Book five).
Augustine thought that demons often were imaginary, but sometimes could enter human bodies, but later accepted the idea of the materialization of demons. Thomas Aquinas followed Augustine's idea, but added that demonic materialization had sexual connotations because demons tried to seduce people to commit sexual sins.
Ambrogio de Vignati, disagreeing with other authors, asserted that demons, besides of not to have a material body could not create it, and all what they seemed to do was a mere hallucination provoked by them in the mind of those who had made a diabolical pact or were "victims" of a succubus or incubus, including the sexual act.
Demons are generally considered sexless as they have no physical bodies, but different kinds are generally associated with one gender or another. Many theologians agreed that demons acted first as succubi to collect sperm from men and then as incubi to put it into a woman's vagina. But as many of them agreed also that demons' bodies were icy, they reached the conclusion that the frozen sperm taken first from a man could not have generative qualities. Albertus Magnus and Thomas Aquinas wrote that demons acted in this way but could fecundate women. Ulrich Molitor and Nicholas Remy disagreed that women could be impregnated; besides, Remy thought that a woman could never be fecundated by another being than a man. Heinrich Kramer (author of the Malleus Maleficarum) adopted again an intermediate position; he wrote that demons acted first as succubae and then as incubi, but added the possibility that incubi could receive semen from succubae, but he considered that this sperm could not fecundate women.
Peter of Paluda and Martin of Arles among others supported the idea that demons could take sperm from dead men and impregnate women. Some demonologists thought that demons could take semen from dying or recently deceased men, and thus dead men should be buried as soon as possible to avoid it.
Diabolical symbols 
According to medieval grimoires, demons each have a diabolical signature or seal with which they sign diabolical pacts. These seals can also be used by a conjurer to summon and control the demons. The seals of a variety of demons are given in grimoires such as The Great Book of Saint Cyprian, Le Dragon Rouge and The Lesser Key of Solomon.
The pentagram, which has been used with various meanings in many cultures (including Christianity, in which it denoted the five wounds of Christ), is sometimes considered a diabolical sign when inverted (one point downwards, two points up). Such a symbol may appear with or without a surrounding circle, and sometimes contains the head of a male goat, with the horns fitting into the upper points of the star, the ears into the side points, the beard into the lowest one, and the face into the central pentagon.
An inverted (upside-down) cross (particularly the crucifix) has also been considered a symbol of both the Devil and the Antichrist, although more traditionally it is the symbol of Saint Peter. See: Cross of St. Peter
Other views 
Not all Christians believe that demons exist in the literal sense. There is the view that the New Testament language of exorcism is an example of the language of the day being employed to describe the healings of what today would be classified as epilepsy, mental illness etc.
See also 
- Classification of demons
- Deliverance ministry
- Demonic possession
- Unclean spirit
- Demons and animals
- Fall of man
- Seven princes of Hell
- Demonologies from Christian and Occultist perspectives
- Thomas Aquinas, Summa Theologica (1274)
- Nicholas Magni, Tractatus de superstitionibus (1405)
- The Sworn Book of Honorius (13th century)
- Johannes Hartlieb, Buch aller verpoten kunst (1456)
- Heinrich Kramer and Jacob Sprenger, Malleus Maleficarum (1486)
- Martin of Arles, Tractatus de superstitionibus (1515)
- Nicholas Remy, Daemonolatreiae libri tres (1595)
- Key of Solomon (16th century)
- Ludovico Maria Sinistrari - De Daemonialitate et Incubis et Succubis (1680)
- The Book of Abramelin (Evidence points to the 18th century, although some claim it to be from the 1450s)
- van der Toorn, Becking, van der Horst (1999), Dictionary of Deities and Demons in The Bible, Second Extensively Revised Edition, Entry: Demon, pp. 235-240, William B. Eerdmans Publishing Company, ISBN 0-8028-2491-9
- Exorcism, Sancta Missa - Rituale Romanum, 1962, at sanctamissa.org, Copyright © 2007. Canons Regular of St. John Cantius
- Hansen, Chadwick (1970), Witchcraft at Salem, p. 132, Signet Classics, Library of Congress Catalog Card Number: 69-15825
- Modica, Terry Ann (1996), Overcoming The Power of The Occult, p. 31, Faith Publishing Company, ISBN 1-880033-24-0
- CATHOLIC ENCYCLOPEDIA: Demonology
- Malleus Maleficarum Part 2, Chapter II, "Now the method of profession is twofold. One is a solemn ceremony, like a solemn vow. The other is private, and can be made to the devil at any hour alone", hosted on the Internet Sacred Text Archive.
- The Witch Persecutions, ed. George L. Burr, p. 3, hosted on the Internet Sacred Text Archive.
- Malleus Maleficarum Part 1, Chapter II, "certain men who are called Lunatics are molested by devils more at one time than at another"; "a man begins to be influenced towards and wills to commit sin, there must also be some extrinsic cause of this. And this can be no other than the devil"
- J. Hampton Keathley, The Beast and the False Prophet (Rev 13:1-18)
- Malleus Maleficarum, Part 2, Chapter II, "But all three kinds have this in common, that though they are very heavy," hosted on the Internet Sacred Text Archive
- Pigments throughout the Ages
- How Demons Enter
- Lewis, James R., Oliver, Evelyn Dorothy, Sisung Kelle S. (Editor) (1996), Angels A to Z, Entry: Incubi and Succubi, pp. 218, 219, Visible Ink Press, ISBN 0-7876-0652-9
- Kramer, Heinrich and Sprenger, James (1486), Summers, Montague (translator - 1928), The Malleus Maleficarum, Part2, Chapter VIII, "Certain Remedies prescribed against those Dark and Horrid Harms with which Devils may Afflict Men", at sacred-texts.com
- The Devil, Satan And Demons