What the Bible makes clear
The Gospels present demons as real, unclean spirits that oppose God and harm people. In Capernaum, an unclean spirit recognised Jesus and obeyed when he commanded it to leave. The people who witnessed it were struck by his authority over these spirits (Mark 1:23-27).
Demons are not God's equals. They do not stand outside his rule, and Jesus never struggles to overpower them. His ministry showed compassion for afflicted people and authority over evil. At the cross, Christ triumphed over hostile spiritual powers rather than being defeated by them (Colossians 2:15).
And they were all amazed, insomuch that they questioned among themselves, saying, What thing is this? what new doctrine is this? for with authority commandeth he even the unclean spirits, and they do obey him.
Why many see demons as fallen angels
Several passages lead many Christians to understand demons as angels who joined Satan's rebellion. Peter writes of angels who sinned and were reserved for judgement, while Jude also describes angels who abandoned their proper place (2 Peter 2:4; Jude 1:6).
Revelation portrays a conflict involving the dragon, identified there as the devil and Satan, and his angels (Revelation 12:7-9). Jesus also speaks of final punishment prepared for the devil and his angels (Matthew 25:41). Taken together, these passages make the fallen-angel view a strong and historic Christian understanding.
For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
Details Scripture does not settle
Still, the Bible never simply says that every demon is a fallen angel. It tells us enough to understand their opposition to God and their defeat under Christ, but it does not offer a step-by-step origin story. There is room here for careful humility.
Some people connect demons with the Nephilim in Genesis 6:1-4 or with later Jewish writings outside the Bible. Genesis raises difficult questions about the sons of God, the daughters of men, and the giants, but it does not explain the origin of demons. Those other ideas may be interesting, yet they should not be treated as certain biblical teaching.
Respond with faith, not fear
Scripture does not invite people to become absorbed with demons. It calls believers to test spiritual claims by their confession of Jesus, resist the devil, and recognise that spiritual opposition is real (1 John 4:1; James 4:7; Ephesians 6:12). The focus is not curiosity about evil, but trustful obedience to God.
If this subject feels unsettling, start with Jesus in the Gospels. He met suffering people with compassion and did not fear unclean spirits. Those who belong to God can take courage because the One at work in them is greater than the evil one in the world (1 John 4:4).
Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.