Exorcism
In Catholic Christianity, exorcisms are performed in the name of Jesus Christ. A distinction is made between a formal exorcism, which can only be conducted by a priest during a baptism or with the permission of a bishop, and "prayers of deliverance" which can be said by anyone.
The Catholic rite for a formal exorcism, called a "Major Exorcism", is given in Section 11 of the Rituale Romanum. The Ritual lists guidelines for conducting an exorcism, and for determining when a formal exorcism is required. Priests are instructed to carefully determine that the nature of the affliction is not actually a psychological or physical illness before proceeding.
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."