What the Bible says clearly
The Bible speaks of demons as evil or unclean spirits. In the Gospels, these spirits bring suffering and bondage, but they are never beyond Christ’s authority. When Jesus met a man in the synagogue at Capernaum, the unclean spirit knew exactly who he was and obeyed his command to leave (Mark 1:23-26).
Scripture also presents Satan as the leader of organised spiritual opposition to God. Jesus referred to “the devil and his angels”, which shows that the devil has angels associated with him (Matthew 25:41). Revelation portrays the dragon, identified as the devil or Satan, being cast down along with his angels (Revelation 12:9).
And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
Why many Christians say yes
Because Satan’s angels are cast down with him, many Christians conclude that demons are those fallen angels. Other passages speak of angels who sinned and did not remain faithful to the place God gave them (2 Peter 2:4; Jude 1:6). Taken together, these verses give a solid biblical basis for that view.
Still, the Bible does not pause to explain the full origin or nature of every evil spirit. It calls them demons, devils, unclean spirits, and evil spirits in different settings. The main point is not to satisfy every question about the unseen world, but to show that evil is real and that God is not powerless before it.
Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
Where Christians differ
Most Christians across many traditions identify demons with fallen angels. A smaller number believe demons and fallen angels may be related but not identical, sometimes drawing on ideas outside the Bible to explain the distinction. Since Scripture does not state the matter in one direct sentence, it is wise to hold the conclusion with humility.
You do not need to solve every detail to trust what Scripture makes plain. Angels were created by God, and some rebelled. Evil spirits resist God’s work. Satan deceives. Yet none of them stands on equal ground with the Creator (Colossians 1:16; Revelation 20:10).
For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him:
Jesus has the greater authority
The Gospels do not treat demons as a curiosity to chase. They point to Jesus. He gave his disciples authority over unclean spirits, and they returned amazed that evil spirits submitted in his name (Luke 9:1; Luke 10:17). Jesus redirected their joy towards belonging to God, not towards spiritual power (Luke 10:20).
So if this question comes from fear, begin there. Christians are called to resist the devil and draw near to God, trusting that God’s strength is greater than evil’s schemes (James 4:7-8). Read the Gospel accounts for yourself, and let Jesus, rather than fear, shape how you understand the unseen world.