Demons are fallen angels and, yes, they do exist (Revelation 12:3–4). Some false gods in the Bible are referred to as demons (2 Chronicles 11:15; Deuteronomy 32:17). The Bible also refers to them as "evil spirits" and "unclean spirits."
Do demons exist?
Demons were initially angels in heaven, but they were cast out when they sided with the Devil over God, becoming his demons instead: "that ancient serpent, who is called the devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him" (Revelation 12:9). The Bible does not provide physical descriptors of demons, but if they were to appear in physical form, they would probably look more appealing and enticing than frightening. The Bible says that there is no truth in Satan, only deception: "even Satan disguises himself as an angel of light" (2 Corinthians 11:14; John 8:44). Satan is a deceiver, through and through, so his demons who followed him out of heaven are deceptive in nature, as well.
Because they are former angels, demons have the same power as angels, though it seems that God has limited them to an extent (2 Thessalonians 2:6–7). Demons primarily exert spiritual influence; however, the presence of demons can manifest in a physical way. For example, in Mark 5, Jesus meets a demon possessed man whose demonic presence in the mind led to him being physically insane. The man could not be physically subdued, even with chains and shackles; his strength was likely enabled by the demons that possessed him. When Jesus delivered the man of the demons that were tormenting him, the man's physical body responded as well, "And they came to Jesus and saw the demon-possessed man, the one who had had the legion, sitting there, clothed and in his right mind" (Mark 5:1–15). The spiritual power demons exert can have physical effects, but that does not mean that all physical ailments or situations are spiritual or demonic in nature (Matthew 10:1; Luke 8:2).
Demons follow Satan and do his work. Just as "your adversary the devil prowls around like a roaring lion, seeking someone to devour" (1 Peter 5:8), demons also look to spread Satan's deception and destruction. This is why, as Christians, we are to: "Put on the whole armor of God, that you may be able to stand against the schemes of the devil. For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places" (Ephesians 6:11–12).
When we recognize that we are in a spiritual battle, we are able to fight against the schemes of the Devil and his demons. We do not need to fear, for when we are in Christ we have power over the Devil, because of the Spirit of God that dwells within us: "Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world" (1 John 4:4).
What are demons according to the Bible?
Are demons and fallen angels the same thing?
What are angels according to the Bible?
Is Satan / the devil a person or some sort of force or personification of evil?
God vs. Satan? If He is really all-powerful, why doesn't God just kill Satan?
Truth about Angels