1. Angelology
Fallen Angels
Part 1

2. Origin and Existence
Scripture does not explicitly state that demons are fallen angels. If they are not, we are hard pressed to explain their origin.
Even the Pharisees in Jesus’ day, acknowledged that Satan was the leader of demons. (Mat. 12:22-24 ) There are three references to the devil and his angels. (Mat. 25:41; Rev. 12:7,9)

3. The reality of the existence of demons is attested by the highest authority. Christ declared their existence on several occasions. (Mat. 12:22-29; 15:22-28; 17:14-20; Mark 5:1-16). He gave His disciples authority to cast out demons. (Mat. 10:1) In sending out seventy preachers by twos He also enabled them to do the same. (Luke 10:17-20 )
Those who deny the existence and power of demons are contradicting Christ.

4. Charles Ryrie, Basic Theology, 183.

5. Descriptions The devil’s angels (Matthew 25:41; Revelation 12:9)
Angels who left their first estate (Jude 6) and angels that sinned (2 Peter 2:4)
Familiar spirits (Deuteronomy 18:11; Isaiah 8:19; 19:3)
Unclean spirits (Matthew 10:1; Mark 1:27; 3:11; 5;13; Acts 5;16; 8:7; Revelation 16:13)
Evil spirits (Luke 7:21; Acts 19:12,13)
Seducing spirits (1 Timothy 4:1)
Wicked spirits (Luke 11:26)
Demon spirits (Revelation 16:14)

6. Nature of Demons
Demons are created persons. They possess intelligence. (Mark 1:24) They tremble before God. (Jam. 2:19)  Demons can communicate and fear the judgment of God. (Luke 8:28) They have a will as demonstrated in the request to enter into a herd of swine. (Luke 8:32). The personal pronouns used in this passage are indicative or personality.

7. Demons are spirit beings. (Mat
Demons are spirit beings. (Mat. 17:18; 8:16; Mark 9:25; Luke 10:17-20) These beings function in a realm different than humans and are therefore invisible. (Eph. 6:12)  
Demons are evil. They are “unclean spirits,” (Mat. 10:1; Luke 4:33)and “evil spirits.” (Luke 7:21) Their wisdom is “devilish.” (Jam. 3:15)
Demons are powerful. This is illustrated by the superhuman strength of the demoniac of the Gadarenes. Another graphic illustration of this power was the foolish actions of the sons of Sceva. (Acts 19:13-16)

8. Demons are religious. Certainly the primary point of their influence is to direct the worship of man away from God and toward any available substitute which is often some form of idolatry. (Lev. 17:7 ; Deu. 32:16,17; Psa. 106:36-38; Rev. 9:20,21)
Demons are very much at work in fostering perverted doctrine. Sometimes it is difficult to understand where some of the doctrinal departures originate. The Bible attributes them to demons. (1 Tim. 4:1-3; 2 Th. 2:2)

9. Perversion is by no means limited to doctrine
Perversion is by no means limited to doctrine. Fallen man is capable of terrible atrocities. Often the despicable sexual activities have their roots in the demons of lust. Violence and bitter hatred can be sourced in demon activity.

10. Demons are numerous. In fact they are “so numerous as to make Satan practically ubiquitous, or everywhere present through their representation. Mary Magdalene was delivered from seven demons. (Mark 16:9) The demoniac was inhabited by “Legion; because many devils were entered into him.” (Luke 8:30) If many demons can inhabit one person (multiple personality syndrome?), there are many demons.