"O unbelieving and perverse generation," Jesus replied, "how long shall I stay with you? How long shall I put up with you? Bring the boy here to me." Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment. (Matthew 17:17-18)
The existence of a supernatural realm is a clear teaching of Scripture. Within the supernatural realm there exists a variety of beings.
Some humans have the desire and ability to influence other humans in such a way that they perform evil actions. It is not unreasonable to suggest that beings who inhabit the supernatural realm may have a similar desire, and an even greater ability. The Bible affirms that this is a reality, and not just a possibility: "When Jesus got out of the boat, a man with an evil spirit came from the tombs to meet him. This man lived in the tombs, and no one could bind him any more, not even with a chain" (Mark 5:2).
Human beings have free will; we can choose to love God, or we can choose to reject God. A similar situation exists in the supernatural realm -- one third of the angelic beings rejected God and chose to follow Satan. These are the beings we call demons or evil spirits.
Humans have very limited access to the supernatural realm. Demons, on the other hand, have a much greater access to the realm in which we function. Their desire, ultimately, is to hinder God's plan of redemption by coercing humans into rejecting the Savior.
Demons have the ability to interact with humans, which includes the ability to take control of a person who willingly yields to them. This is known as demonic possession. Christians are indwelt by the Holy Spirit -- they cannot be demon possessed: "Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God?" (1 Corinthians 6:19). Christians can, however, be demon oppressed. When oppression comes, the Christian is to resist: "Submit yourselves, then, to God. Resist the devil, and he will flee from you" (James 4:7).
Many cults deny the existence of demons, though most affirm the existence of a supernatural realm. To categorically deny the possibility of the former, and yet affirm the reality of the latter, is irrational. Historic Christianity affirms both.
"For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand" (Ephesians 6:12-13).