Exorcism
In Catholic practice the person performing the exorcism, known as an exorcist, is an ordained priest. The exorcist recites prayers according to the rubrics of the rite, and may make use of religious materials such as icons and sacramentals. The exorcist invokes God—specifically the Name of Jesus—as well as members of the Church Triumphant and the Archangel Michael to intervene with the exorcism. According to Catholic understanding, several weekly exorcisms over many years are sometimes required to expel a deeply entrenched demon.
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."