What are demons?

If you believe the Bible, then you must believe that demons are real. According to the Bible demons where once heavenly angels, however they choose to sin against God and to follow Satan. The consequences of this choice led to them being cast out of heaven.

And the great dragon was thrown down, 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. (Rev 12:9 ESV)

Demons like angels do have different classes. There are principalities, powers, rulers of darkness, spiritual wickedness.

For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. (Ephesians 6:12 KJV)

Like Satan, demons should NOT be directly interacted with. They wish to possess people and cause harm to them.

Then it goes and brings with it seven other spirits more evil than itself, and they enter and dwell there (in a person), and the last state of that person is worse than the first. So also will it be with this evil generation.” (Matthew12:45 ESV)

And it (a demon) has often cast him (a boy) into fire and into water, to destroy him…” (Mark 9:22 ESV)

Demons possessing people is not just from biblical times. Demons are still very much around in modern times and wish to possess people to harm them and do evil. If you do not believe so, then watch this interview with a Catholic Exorcist. Bone-Chilling Interview with a Real-Life Exorcist (Fr. Vince Lampert) or watch the documentary movie The Devil and Father Amorth.

Satan is the leader of the demons, and he uses demons to carry out this will and perform evil tasks. However, we know that Satan and his demons will share the same fate.

…into the eternal fire prepared for the devil and his angels. (Matthew 25:41 ESV)