Christian demonology is the study of demons from a Christian point of view. It is primarily based on the Bible (Old Testament and New Testament), the exegesis of these scriptures, the scriptures of early Christian philosophers and hermits, tradition, and legends incorporated from other beliefs.
Like theology, demonology is not a science; therefore, strict[scientific method]has not always been applied.
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 have been attributed 'evil' qualities by the Christian churches.
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 mainly studied within the Roman Catholic Church, although some other Christian churches do not deny the existence of demons.
According to Christian tradition, demons can be angels, are spiritual, immutable and eternal. Demons are not [omniscient, but each one has a specific knowledge (sometimes on only one subject, sometimes on more than one). 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 Devil can be in different places at the same time, or he sends an emissary in his name, but it seems that the only 'omni' thing they are is malevolent; it is deducible from some passages of the Bible that they are not omnipresent.
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. Demons have also the duty of punishing the souls of those people that died out of God's grace (in sin), by torturing them in Hell.
It is also believed that demons torment people during their life, like the case of Job or through possession, causing disgraces and diseases, or simply showing themselves before persons to frighten them, or by provoking visions that could induce people to sin or to be afraid. (Luke] 13:16; 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 adversaries of God. (Ephesians 6:12)
Demonic supernatural powers are believed to include Fabrication, Psychokinesis, Pyrokinesis, Levitation, Cryokinesis, Biokinesis, Divination, Demonic possession, seducing spirits, ESP, telepathy, mind probing, witchcraft, curses and hexes, as well as binding, magnification, and making contracts, weather control, animal control, and provacation]. Demons use variants and combinations of these powers to harass, demoralize, confuse, and disorient the victim, or the willing subject of demonic interest. Many of these attacks can be limited by God for duration, effect or scope of these Demonic attacks. A fictional example of the above is featured in the movie The Exorcist.
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, however, 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, wrecking havoc, ensuing chaos, and disrupting peace.  
According both to Christian theology and Christian demonology, all evil in the world is caused by demons with God's permission, as their mission serving God; for example in the narratives of the Book of Job and the Temptation of Christ.
According to the Book of Revelation (Rev 12:9), demons are the angels that fell from heaven with Satan when he chose to rebel against God.
Justin Martyr, a Christian who lived circa 150 AD, believed (based on Gen 6:2-4) that demons were the offspring of angels and men. The Greeks viewed demons as the spirits of wicked dead men (Heb 9:27).
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.
Referring to their appearance, demons can take any desired appearance, even that of an "angel of light" (2 Corinthians 11:14). 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, Francisco Goya, the artist that made the drawings for the Dictionnaire Infernal, and others.
However, according to Nicholas Remy, the figure of the demons is imperfect, which he deduced from the descriptions given by those accused persons interrogated during the witch trials; he wrote:
- "...that proves how marvellous God's love is, even for the most miserable human beings, being that demons can never take a human figure in a perfect form, and so the most stupid people are able to discover them".
The idea that demons have horns seems to have been taken from the Book of Revelation 13:1 (here it seems that John the Evangelist was inspired by Leviathan) and 13:11. The book of Revelation seems to have also inspired some absurd depictions of demons (Revelation 13:1-2). This idea can also been associated with the depiction of certain ancient gods like Baal, Moloch, the shedu, etc, which were portrayed as bulls, as men with the head of a bull, or wearing bull horns as a crown.
While there is no conclusive theory about why the Devil appears with a tail, the trident attributed to him as his "sceptre" may have been inspired by early contact between Christians and Hindus. In Hinduism, the trident is Shiva's weapon.