Yes, demons exist, as evidenced by their mention in both the Old and New Testaments. Demons are often referred to as "evil spirits" (Luke 7:21) and "unclean spirits" (Mark 1:23) and are described as fallen angels, who followed Satan’s rebellion against God (Revelation 12:9). Demons can exert significant spiritual influence and, in some cases, take a physical presence, as demonstrated in the story of the demon-possessed man in Mark 5:1–15. Although demons can oppress and deceive, they cannot possess a Christian, who is protected by the power of Christ (1 John 4:4). Believers can find confidence that God has conquered Satan and his demons and that He has given us all we need to identify and counter the schemes of demons.
Demons want to steal, kill, and destroy, just like Satan, whom they follow. However, Christ has defeated Satan and his angels, and one day, they will be thrown into the lake of fire. In the meantime, we are to be wise and know the demons’ schemes and deceptions and counter them with spiritual weapons: "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). We can use the truth of God’s Word, righteous living, and our salvation to counter the attacks of demons. And, although demons can oppress a Christian, that is, bother them and try to make them sin and fall, they cannot possess a Christian. When we recognize that we are in a spiritual battle, we are able to trust in God and 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).