Demons (Mark 1:21-28; Luke 10:17-20; 11:14-26; 1 Corinthians 10:14-24; 1 John 4:1-6) are supernatural beings that are almost immortal, and were created many eons ago before the Fall (Isaiah 14:12-14; Daniel 10:13; Ezekiel 28:12-15(the OT passages are figurative); John 12:31; Eph. 6:12; 2 Pet. 2:4; Jude 6). The word for Demons (daimon/daimonia) is from the Greek, a broad term meaning spiritual beings or small gods (Acts 17:18). Elsewhere in the NT, the word for demons refer to evil spirits (Matt 8:16; Luke 10:17, 20; Matt 17:18 Mark 9:25). They were once angels. A third of them sought pride and rebellion against God, so they joined with Satan to fight God. They lost and fell from their state of grace, thus becoming demons. They are basically fallen angels who are totally depraved (evil) in contrast to the angels who are still in a state of harmony with God and grace, and who are also sinless because they did not fall with Satan.
Satan and his demons have an incorporeal nature meaning they are immaterial-- having no physicality, such as a body (Eph. 2:2; 6:12; Rev 16:14). Since they are purely spiritual, they do not have to function in our corporeal (material reality) realm.
There are two classes of demons, those who are free to roam the earth and harass people (Eph. 2:2; 6:11-12; Col. 1:13), and those who are imprisoned in the temporary prison of the abyss (Luke 8:31; Rev 9:1-11; 20:1-3). They will be judged at the Day of Judgment (Matt. 8:29, 2 Pet. 2:4; Jude 6). They will be punished (Matt. 8:29; 25:41; Luke 8:28; 2 Pet 2:4; Jude 6; Rev 12:7-9). They know they are doomed to destruction (Matt 8:29). Their final fate will be the lake of fire (Matt 25:41), the eternal dwelling of Satan, demons, and all unsaved people.
They are adversaries of humanity (Matt. 12:45), sent to cause trouble between peoples, destroying relationships, and causing us to fall away from God (Judges 9:23). Even they know that they are doomed, so they spend all of their energies seeking to destroy humanity and God's reputation, to take as many with them as possible.
They seek to corrupt us with false doctrine (1 Kings 22:21-231; Tim 4:1-5), and obscure the truth of who Christ is (James 2:19; 1 John 4:1-3;). They can affect people, including Christians, morally and mentally (Luke 8:27; 2 Tim 3:1-9; Rev 9:20-21; 16:13), and effect non-Christians physically with maladies such as blindness (Matt 12:22), insanity (Luke 8:26-36), stupidity (Matt 9:32-33), and suicidal tendencies (Mark 9:22). However, as Christians, we can be harassed but not possessed (1 John 4:4).
They also possess supernatural knowledge that we do not have access to. They know firsthand about redemption and Christ (Mark 1:24), but reject it. We do not have firsthand knowledge, but by faith we receive it. They can influence people with their knowledge, and convince people they know the future, which they do not. They can only make good presumptions. They can make their victims exhibit superhuman physical strength (Luke 8:29). They are invisible and are not limited to anything physical such as walls or people. However, they may assume, or appear in human form (2 Kings 2:11; 6:17; Rev 9:1-12; 16:13-16).
They operate by being shrewd. The demons in Matthew, chapter 8, begged Jesus not to vanquish them. In pagan literature, they liked to negotiate the least difficult terms to their favor, just as attorneys do today (I am not saying this as a "put down," but as an example).
The demons recognized Jesus for who He is in power and position, and they are one of the greatest testimonies to the deity of Jesus (Matt. 8:29; Mark 1:23-24; 3:11; 5:7; Luke 8:28; Acts 19:15). They have belief but not faith! In Matthew 8:28-34, they responded to Jesus with hate, tempered with fear. They knew who He was, and they knew their fate to come in judgment and destruction.
The corrupt Canaanites and other evil people groups would worship them: (Lev. 17:7; Duet. 32:17; 2 Chron. 11:15; Psalm 106:37; Matt. 4:9; Luke 4:7; 1 Cor. 10:20-21; 1Tim. 4:1; Rev. 13:4). This worship of demons and anything that is not God is strictly forbidden and will destroy us: (Lev. 17:7; Zech. 13:2; Rev. 9:20).
Instances of people being possessed or affected include Saul, (1Sam. 16:14-23; 18:10-11; 19:9-10), two demon possessed men (Mt 8:28-34; Mk 5:2-20), the disabled (Matt. 9:32-33; 12:22; Luke 11:14), and Mary Magdalene (Mk 16:9; Luke 8:2-3). Other instances are found in Matt. 9:17-27; 15:22-29; 17:14-18; Mark 1:23-26; 7:25-30; Luke 4:33-35; and 9:37-42.
Even Jesus was falsely accused of being possessed by demons in Mark 3:22-30; John 7:20; 8:48; and 10:20.
There are Instances of people being exorcised by Jesus (Matt. 4:24; 8:16; Mark 3:22; Luke 4:41), and by the disciples (Matt. 10:1; Mark 6:7; 9:38; 16:17; Luke 10:17; Acts 5:16; 8:7; 16:16-18; 19:12-16). Examples of the disciples inability to drive out demons are found in Mk 9:18, 28-29.
If you take the Bible seriously, you also must take into account the veracity and reality of demons! They are not figurines of a bygone belief system, which is what they want you to think. They are real, and are after you! The comfort we have is that He (Jesus) that is in us is greater than he (Satan) that is in the world, so they cannot do anything to you not allowed by God. At the same time, we are called to stand firm against them (2 Cor. 10:3-6; Eph. 6:10-18; Col. 2:15; 1 John 4:4). Please see our article on the "Names of Satan" for more information.