The subject of mental illness is very controversial in Christian circles. Inside the extreme schools of thought, we find balance and a scriptural viewpoint.
First, let me say clearly: All mental illness is not the result of demonic attack. Further, good psychological care from Christian professionals is vital and in order when an individual is struggling.
Also, professionally administered medication may be necessary when chemical imbalances occur. But when normal medicine and therapy do not result in a cure, then it is possible that these symptoms could point to demonic operation.
Let me list for you 14 symptoms of demonic operation. As these symptoms are listed, realize that some of them could also be caused by something other than demonic oppression. The first six symptoms on this list are extreme and are drawn from the account of the demoniac of Gadara in Mark 5. This man had been chained in a cemetery because of his erratic and violent behavior. When you look at Mark 5:1-15, you can see clear symptoms of demonic activity.
Symptom 1: Incapacity for normal living (Mark 5:1-5)
Just as the actions of Legion made the man unsuitable for normal social interaction with friends and family, an unusual desire for solitude, accompanied by a deep loneliness, will often set in. The person will often become very passive with no desire to change.
Symptom 2: Extreme behavior (Mark 5:4)
Violence will often be evident in the victim’s life. An explosive temper and extreme, uncontrollable anger are dangerous behaviors that control the individual and the people who love him or her.
Symptom 3: Personality changes (Mark 5:9, 12)
Multiple personalities exist in some of the most serious cases of demonic control. This man had a “legion” of spirits within his life. All cases of multiple personality may not be demonic, but in most cases demon activity is involved. Changes in personality, extreme or mild, may be evidence of demonic activity.
Symptom 4: Restlessness and insomnia (Mark 5:5)
In verse 5, we see this man crying in the tombs “night and day.” He could not sleep. Insomnia can be a sign of a physical problem or a sign of a spiritual problem. God has gifted His children with sleep (Ps. 127:2). So when you cannot sleep night after night and there is no medical reason for this disturbance, the devil maybe tormenting you. Don’t forget: You have the right to rest in Jesus!
In Psalm 3, we see a picture of warfare. Here David was hounded by his enemies. In verse 3, he cried to the Lord, “You … are a shield for me.” In verses 5 and 6, he cried, “I lay down and slept; I awoke, for the Lord sustained me. I will not be afraid of ten thousands of people who have set themselves against me all around.” He also said in Psalm 4:8, “I will both lie down in peace, and sleep; for You alone, O Lord, make me dwell in safety.” Sleep is God’s gift to all who trust in Him.
Symptom 5: A terrible inner anguish (Mark 5:5)
This man was deeply tormented in mind and heart. Various levels of anguish are evident in those who are afflicted by demons. Grief and anguish are normal emotions for us all. Yet persistent, unresolved anguish that will not leave after normal therapies of counseling, encouragement and prayer could well be demonic.
Symptom 6: Self-inflicted injury and suicide (Mark 5:5)
Here we see the demonic man cutting himself. If you read Mark 9:14-29, you will see the story of the man whose son was both deaf and mute because of a demon: “Wherever it [the evil spirit] seizes him, it throws him down … Often he [the demon] has thrown him [the boy] both into the fire and into the water to destroy him” (vv. 18, 22). Jesus cast out the demon: “The spirit cried out, convulsed him [the boy] greatly, and came out of him. And he became as one dead … But Jesus took him by the hand and lifted him up, and he arose” (vv. 26-27). Demons can cause people to injure themselves. They even incite suicide.
Symptom 7: Unexplained illness with no obvious medical cause
When medical testing produces no physical cause for an illness, then we should look to the mind and spirit for answers. Sometimes illnesses are psychological, and good counseling can result in a cure. Other times the battle is with demons. A scriptural example of this is found in Luke 13:11-16, the story of a woman afflicted by a “spirit of infirmity” (v. 11). Jesus called her “a daughter of Abraham, whom Satan has bound” (v. 16). Obviously she was a child of God and faithful to her synagogue, with a desire to know more about the Lord. Then, “Jesus … said to her, ‘Woman, you are loosed from your infirmity.’ And He laid His hands on her, and immediately she was made straight, and glorified God” (vv. 12-13). There are physical illnesses caused by a class of demons known as “spirits of infirmity.”
Abby Trivett 
James Lasher