When a demon enters a living human body and gains control over it, the resulting condition is called Possession. It can often be recognized by abrupt changes in a person's behavior and abilities. For example, the new condition may lead to fits and convulsions, erased memories, unusual physical strength, ability to predict the future, speaking in ancient languages, fear of crosses and holy relics, mental illness, strange facial contortions, and changes in the tone of voice.
One of the most noteworthy signs of possession is exceptional physical strength. In one case a small thin woman became so strong that she easily broke free when tied up with ropes, and three large men were required to hold her down. Another example is reported in the bible in a passage at Mark 5: 1-4, which describes a possessed man who "lived in the tombs, and no one could bind him any more, not even with a chain. For he had often been chained hand and foot, but he tore the chains apart and broke the irons on his feet. No one was strong enough to subdue him." Another case is reported in Acts 19: 16, which describes a fight in which a demon-possessed man completely over-powered seven other men simultaneously.
Another common indication of demonic possession is mental disorder. The victim may seem confused and dazed, talk incoherently, and behave strangely. In one reported case a patient rapidly blinked his eyes, breathed heavily, and constantly chewed his lips. Sometimes the condition will resemble psychological illnesses such as schizophrenia, appearance of an alternate personality, and hysteria. Some people believe that demons are also responsible for cases of alcoholism, sloth, addiction, proneness to violence, and adultery. In some ancient societies spirits were thought to cause epileptic seizures.
Note: The painting shows an expelled demon leaving a victim. The image is a public domain work taken from a medieval book called Les Tres Riches Heures du duc de Berry, which is a "Book of Hours" that was created for the Duc de Berry.
The most common treatment for possession is to attempt to compel the demon to leave the body of the victim. Individuals who attempt such treatments are known as exorcists, and a successful expulsion is called an exorcism. The procedure can take considerable time, often several days. Sometimes the victim, controlled by the demon, will actively resist, and may have to be tied down on a table or bed. Christian exorcists typically try to induce the demon to leave the body by prayers, and by calling on God, Jesus, and various angels for help. In primitive societies shamans use incantations, ritual gestures, and special objects or idols to bring about the cure.
The demon will usually resist the attempt to drive it from the body. This resistence will be visible in the struggles of the victim, who will jerk, scream, shake, and try to break free from any restraints. When the evil spirit finally leaves, the victim may give one last terrifying shriek, then collapse in total exhaustion.
The bible describes several cases in which Jesus performed exorcisms. For example, in Matthew 9: 32-34, he restores the ability to speak to a man made mute by possession, and in Mark 5: 1-20 he expels many demons from a single man. Jesus had so much spiritual power that the demons never even tried to resist him, and always left the victim without delay. To learn more about this, go to Information about demon possession. This reference also contains an interesting image showing Jesus expelling a demon.
Ancient stories indicate that demons were initially angels residing in heaven, but that they joined the Devil in his reckless attempt to take the place of God as the supreme ruler of the universe. After Satan's revolt was defeated, the angels who helped him were expelled from heaven and transformed into the demons that now exist. According to some accounts, a third of the original angels took part in the revolt and thus ended up as demons. Nobody knows how many this would be, but the total number could run into millions. Dr. Johann Wier (1515-88) determined from his research that there are exactly 7,409,127 demons altogether, and that they are led by 79 individual demon princes. But other epople have arrived at different estimates.
Artists sometimes depict demons as hideous beast-like creatures similar to gremlins. They are often given horns and black skin. But since they are spirits, it is easy for them to disguise themselves or even become invisible. They may even take the form of a handsome man or beautiful woman in order to trick people. Some reports indicate that they feel cold and icy when touched, so this could be a way to identify them. According to some sources, if you spread ashes around your bed at night, you might be able to see demon footprints in them in the morning, showing that they are watching you.
Some ancient religious prophesies predict that eventually the world will enter a period known as the End Times, during which Satan and his demons, representing the forces of Evil, will engage in a great war against the forces of Good. After a desperate struggle, the forces of Good, led by Jesus, will finally be victorious. As punishment for their evil ways, Satan and all the demons will be thrown into a Lake of Fire at the center of hell where they will have to endure a fiery pain for all eternity.
Oddly enough, some people have attempted to use demons for their own evil purposes. Such individuals may employ rituals and magical symbols to try to call a spirit into their presence so that they can ask it to carry out some task that only a spirit can perform. However, this may be dangerous, and it's even possible that the spirit will decide to take possession of the person who called it. Demons obey the commands of higher powers such as God, Jesus or the Devil, but they don't feel any obligation to obey human beings. They should be viewed as enemies of people, not as possible helpers, so it is best to avoid them.
Of course there are many people who regard belief in the existence of demons as nothing but a primitive superstition. Doubters also say that the idea of possession is merely a survival from earlier times when there was little knowledge of medicine and psychology, so that other explanations for strange behavior needed to be found.