The Nature and Work of Satan
Both the Old and New Testaments repeatedly acknowledge the existence of Satan. Scripture teaches that he is a personal being who possesses intellect, emotion, and will (Zechariah 3:1-2; Luke 4:1-13; 2 Corinthians 11:3; Revelation 20:7-8). He was created by God as an angel (Isaiah 14:12-13; Matthew 25:41; Revelation 12:9), and as “the anointed cherub,” he was originally perfect in beauty and blameless in his ways until his rebellion against God (Ezekiel 28:12-15). He is evidently the most powerful creature God ever made (Jude 9). His heart was lifted up because of his beauty; his splendor corrupted his wisdom, and he raised himself up against God in an act of titanic self-assertion (Ezekiel 28:16-17; Isaiah 14:12-15 [while these prophetic passages refer to historical characters, it appears that the poetry also uses these figures to allude to a cosmic dimension of spiritual evil]).
Because of his rebellion, Satan’s character was completely distorted, and he became a powerful force of evil in the universe. He has dominion over a vast army of fallen angels and rules over the world system. His titles and names reveal his position, power, and practices.
The Nature and Work of Demons
The Bible amply supports the existence of demons. They are found in the Old Testament (Leviticus 17:7; Deuteronomy 32:17; Psalm 106:37) and in the teaching of Christ and the apostles (Mark 1:23-27; 1 Timothy 4:1; 2 Peter 2:4). In the gospels, demons are called “unclean spirits” (Mark 7:25-26) and “evil spirits” (Luke 8:2) as well as the angels of the devil (Matthew 25:41). The word for demon (daimonion) does not mean, “devil.” That word comes from diabolos, “slanderer,” a name for Satan. Evidently, demons are the angelic beings who joined with Satan in his rebellion against God (Matthew 25:41; Jude 6; Revelation 12:4). Satan is the “ruler of the demons” (Matthew 12:24), and they are organized to accomplish his plan of overthrowing the kingdom of God (Ephesians 6:11-12; Revelation 12:7).
Like Satan, demons are personal beings; they are not superstitions, impersonal forces, or psychological states. They have intelligence (Mark 1:23-24; Luke 8:27-33), emotion (Luke 8:28; James 2:19), and volition (Matthew 12:44; Luke 8:32). They differ in rank and power (Ephesians 6:12) and also in degree of wickedness (Matthew 12:45). Some demons appear to produce foolishness and nonsense; others seek to degrade and destroy.
Taken from Ken Boa's Handbook to Spiritual Growth.