16.1 What are devils and unclean spirits?
ANSWER:
In the Bible, illnesses - especially mental illnesses - are called "devils", or "demons", or "evil spirits", or "unclean spirits". When the people were healed the illness - "unclean spirit" - was said to be "cast out". Sometimes an "unclean spirit" means "wickedness".
See 1 Samuel 16: 14; Matthew 8:16-17; Matthew 12:22.

16.2 Pagans believe that "evil spirits" are invisible living beings. Should a Christian believe this?
ANSWER:
No. Some churches teach this pagan belief, but it is not true. The Bible does not teach it. The Bible says that God is the Master of His own universe. There are no living spirit beings except God, the Lord Jesus, and God's angels. The angels do God's will perfectly, so they can neither sin nor die. As we have already seen, the devil, Satan, and evil spirits are used in the Bible as a kind of parable. They represent sinful human nature, and illness, which is a consequence of our sinful state.
See Isaiah 45:5-7; Amos 3:6; Matthew 6:10.