One relatively recent example of Christian demonology is the book War On The Saints, by Jessie Penn-Lewis, which purports to reveal what demons are, how they work at gaining possession, how to recognize their workings, how to oppose them, and how to free oneself and others from demon possession.
Since early Christianity, demonology has evolved 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.
St. Albertus Magnus said of demonology, "A daemonibus docetur, de daemonibus docet, et ad daemones ducit" ("It is taught by the demons, it teaches about the demons, and it leads to the demons").
According to Christian tradition, a demon is an evil spirit, and can be either a fallen angel or the spirit of a condemned human, and its intent is to lead mankind into sin.
Tradition says that demons do the most damage when they are given "ground" for their workings, i.e. when they are accepted, consciously or unconsciously, knowingly or unknowingly, by a person. Acceptance is often construed as committing sin; demons gradually gain control of the eyes when the eyes look upon sinful things, gain control of the tongue when it is used for slander or blaspheme. Another way demons can be accepted by their human host is to tell lies that the human believes or give suggestions, subtly disguised as the host's own thoughts. This phenomenon is called possession. Examples of are given in the Gospels.
Alleged victims of demonic possession may hear voices, becomes obsessed or fixated on an object or idea, develop a split personality, be impervious to reason, develop a hatred of God, or become interested in bestiality or pedophilia. These symptoms are usually signs of mental illness, not possession, and should be treated by a licensed medical professional.
Forbidden rituals, including the 'Ritual E' Damo', the study of magic (both black magic and white magic, as both ultimately come from demons and are believed to lead to ruin), worshiping false gods, accepting visions received from evil spirits (mediums and diviners), and having faith that these visions will come to pass.
Believers say that to protect against demonic possession, one must have faith in God, an alert watchfulness, a guarding of one's own mind, and a spoken refusal of the influence of all evil spirits. Prayer against the evil spirit or spirits, prayer to shed light on what action(s) was done or word(s) said that allowed them ground for possession so that this too can be refused are also often necessary.
According to the apocryphal Book of Enoch, 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’. (Enoch15v8-12, 16v1. C.H.Charles.)
In John 8:44 Jesus calls the Devil "the first homicide" ("he was a murderer from the beginning" in the King James Version), perhaps referring to the murdering of Abel by Cain, a liar, and father of all lies.
In early times of Christianity it was accepted an unknown number of demons, but later, during the Late Middle Ages, some demonologists tried to evaluate this number.
Some scholars did not accept a unique number of demons. Gregory of Nyssa, in the 4th century, supported the idea that demons procreated with other demons and with women, believing in the existence of male and female demons.
Other scholars supported the idea that the number of demons was unique and they could not procreate.
As it had been said by Christian theology that there were 400 million angels, Alfonso de Spina calculated that about one third of them were demons, and in 1467 he asserted that the number of demons was 133,316,666 demons. This idea of 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 in 44,435,622, divided in 666 Legions, each legion composed by 6,666 demons, and all of them ruled by 66 hellish dukes, princes, kings, etc. He forgot to add the rulers of these rulers to the total, but not to mention them in his book. Besides, the number of legions mentioned by Weyer varies in different editions of his book.
The Lesser Key of Solomon (17th century) copied the division in legions from Pseudomonarchia Daemonum but added more demons, and so more legions; anyhow, its anonymous author did not calculate the number of demons cited in this work. According to the editions of this book, the number also varies.
It is suggestive that both Spina and Weyer used the 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).
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, like the case of Job or through possession, causing disgraces or simply showing themselves before persons to frighten them, or by provoking visions that could induce people to sin or to be afraid. (Matthew 17:15-16)
Demons are also believed to try to make people abandon 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)
The idea that demons have horns seems to have been taken from the Book of Revelation 13:1 (here it seems that John was inspired by Leviathan) and 13:11. The book of Revelation seems to have also inspired some depictions of demons (Revelation 13:1-2). 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 affirmed that they were heavier than common humans.
About the colour 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.
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 anything 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. Demons, assumably, are granted permission to test, bring about trials, and to tempt people through the use of their destructive powers, to make people prove their faith, sometimes as a means to carry out the will of the Lord. Often Demons are said to creating 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 demon 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 had to manifest themselves in a visible and if possible tangible form.
Basil of Caesarea was, apparently, the first who wrote on this subject. He believed that demons, to materialise, had to condense vapours 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 vapour to form them (see below). Many authors believed that demons could assume the shape of an animal, preferably black.
It seems that until the first millennium, when the fear for the coming of the Antichrist reached proportions that were out of control, the appearance of demons was not a significant problem. But since this moment on, demons acquired a terrible appearance in the mind of those who believed to have seen them.
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 materialisation of demons. Thomas Aquinas followed Augustine's idea, but added that demonic materialisation 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.
The demons' buttocks, genitalia and sperm were a subject of dedicated study by Christian theologians, demonologists and inquisitors. The Inquisition seemed to have been particularly interested in this topic.
Concerning the demons' behind, and lever testicals, there were confessions asserting that they were normal, others telling that instead of an anus they had another mouth and thus when kissing their behind during the Sabbath people received another kiss in exchange, and confessions telling that demons did not have buttocks.
About the demons' testicles, only one witch confessed to Pierre de Rostegny that the demon with whom she had sexual relationships had them. Other confessions denied that demons had them. Henri Boguet supported the idea that demons did not have sexual organs and Johann Meyfarth asserted that demons did not have a penis.
The demons' penis was a problem for inquisitors and scholars. Many of them manifested a morbid interest in the demons' genitalia, but the penis reached pathological proportions. Many questions during the interrogatories in the witch trials referred to this theme. All persons confessed to have had sexual relationships with at least one demon, but the descriptions given of this particular part of their anatomy vary from a small phallus to a big one. Some confessions described a normal penis in the appropriate place, others a normal one in the behind, others two phalluses, one in its place and the other in the behind, and others a bifid one, like the tongue of a snake. Confessions that described two phalluses or a bifid one often added the particularity that the demon practised vaginal and anal coitus at the same time; Sylvester Prieras was a supporter of this idea. Even some confessions described three penises. In one legend, it was described to be six-foot long. Concerning the material of which it were made, there were confessions affirming that it was normal and flesh-made, others saying that it was iron or horn-made, others telling that it was half flesh and half iron, and others saying that it had scales and, being scaly, the sexual act was painful; even some confessions asserted that it was bone or wooden-made.
The sperm of the demons constituted another problem. Some persons confessed that this sperm was icy, meanwhile others felt it as that of a common man. But another problem arose among scholars to determine if demons had their own sperm or not. Ludovico Maria Sinistrari was one of the few authors that supported the idea that demons were corporeal entities that had their own sperm and with it could impregnate women and conceive children with them. But most scholars denied the idea that demons could have their own sperm, and concluded that they took sperm from men. The problem grew when these authors had to explain how demons took that sperm, how did they put it into a woman's vagina, and if that sperm could conceive children or not.
Most theologians agreed in the fact that demons acted first as succubae to collect sperm from men and then as incubi to put it into a woman's vagina. But as many of them agreed also in the fact 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 in the fact that women could be impregnated; besides, Remy thought that a woman could never be fecundated by another being than a man. Heinrich Kramer and Jacob Sprenger (authors of the Malleus Maleficarum) adopted again an intermediate position; they wrote that demons acted first as succubae and then as incubi, but added the possibility that incubi could receive semen from succubae, but they 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.
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.