Exorcism
In Christianity, exorcism is the practice of casting out demons. In Christian practice the person performing the exorcism, known as an exorcist, is often a member of the Christian Church, or an individual thought to be graced with special powers or skills. The exorcist may use prayers and religious material, such as set formulas, gestures, symbols, icons, amulets, etc. The exorcist often invokes God, Jesus and/or several different angels and archangels to intervene with the exorcism. Protestant Christian exorcists most commonly believe the authority given to them by the Father, Son, and Holy Spirit (the Trinity) is the source of their ability to cast out demons.
Satan
Most early Christians firmly believed that Satan and his demons had the power to possess humans and exorcisms were widely practiced by Jews, Christians, and pagans alike. Belief in demonic possession continued through the Middle Ages into the early modern period. Exorcisms were seen as a display of God's power over Satan. The vast majority of people who thought they were possessed by the Devil did not suffer from hallucinations or other "spectacular symptoms", but "complained of anxiety, religious fears, and evil thoughts."