Demons are evil spirits that are opposed to God and God’s people.  They appear to be fallen angels:  2 Pet. 2:4 & Jude 6 are primarily referenced in the New Testament.

• Matt. 4:24, “And the news about Him went out into all Syria; and they brought to Him all who were ill, taken with various diseases and pains, demoniacs, epileptics, paralytics; and He healed them.”
• Matt. 8:28, “And when He had come to the other side into the country of the Gadarenes, two men who were demon-possessed met Him as they were coming out of the tombs; they were so exceedingly violent that no one could pass by that road.”
• Mark 3:22, “And the scribes who came down from Jerusalem were saying, ‘He is possessed by Beelzebul,’ and “He casts out the demons by the ruler of the demons.”
• Luke 4:35, “And Jesus rebuked him, saying, “Be quiet and come out of him!” And when the demon had thrown him down in their midst, he came out of him without doing him any harm.”
• John 8:48-49, “The Jews answered and said to Him, “Do we not say rightly that You are a Samaritan and have a demon?” ⁴⁹ Jesus answered, “I do not have a demon; but I honor My Father, and you dishonor Me”

Demons are spiritual beings without physical form.  They are evil spirits that are opposed to God and his work.  They were probably created before the world was and, like Satan, fell away from God. Some believe that Rev. 12:4 is a reference to a third of the angels falling and becoming demons.

The term demon comes from the Greek “daimon.”  They are found in the New Testament as being able to possess people (Matt.  8:28-34; Mark 5:1-4) and animals (Matt. 8:32).  They are very strong (Mark 5:4).  They are also called unclean spirits (Luke 8:29), seem to have a hierarchy of rulership in their demonic realm (Matt. 12:24-27), and can have sacrifices offered to them (1 Cor. 10:20).  Also, they will be judged in the future (Matt. 8:29).  They cannot be redeemed.

Paul recognized demonic forces and referred to them as principalities (Rom. 8:38) and said that we struggle against them (Eph. 6:11-12).  He speaks of them in the heavenly places, but this does not mean heaven where God is but the upper area above the earth.

Jesus was able to cast out demons from the demon-possessed people (Matt. 8:32) and was accused of being demon-possessed (John 7:20; 8:48).