Revelation  12:9 is the clearest scripture on the identity of demons, “The great dragon  was hurled down—that ancient serpent called the devil, or Satan, who leads the  whole world astray. He was hurled to the earth, and his angels with him.”  Satan’s fall from heaven is symbolically described in Isaiah 14:12-15 and Ezekiel  28:12-15. Revelation  12:4 seems to indicate that Satan took one-third of the angels with him when  he sinned. Jude 6 mentions  angels who sinned. The Bible indicates that the demons are fallen angels who,  along with Satan, rebelled against God.

Satan and his demons now look to  destroy and deceive all those who follow and worship God (1 Peter 5:8; 2 Corinthians  11:14-15). The demons are described as evil spirits (Matthew 10:1), unclean  spirits (Mark 1:27),  and angels of Satan (Revelation  12:9). Satan and his demons deceive the world (2  Corinthians 4:4), attack Christians (2  Corinthians 12:7; 1 Peter  5:8), and combat the holy angels (Revelation  12:4-9). Demons are spiritual beings, but they can appear in physical forms  (2  Corinthians 11:14-15). The demons/fallen angels are enemies of God, but they  are defeated enemies. Greater is He who is in us, than those who are in the  world (1 John  4:4).