Are some forms of mental illness really demon possessions?
Some forms of mental illness are indeed demonic possession in that the personality of the demon manifests itself in the place of the person being possessed. Remember the demonic man of Gadara in the Scriptures. He had incredible strength so as to break metal chains and shackles placed on him, he cut himself with stones and dwelt among the tombs. But after Jesus cast out the legion of demons (a Roman legion consisted of up to 6,000 soldiers), the Scriptures said the man was coherent and in his rightful mind. Luke 8:35-36 35 Then they went out to see what was done; and came to Jesus, and found the man, out of whom the devils were departed, sitting at the feet of Jesus, clothed, and in his right mind: and they were afraid. 36 They also which saw it told them by what means he that was possessed of the devils was healed. Granted, some forms of mental illness are caused by physical factors; Matthew 4:24 24 And his fame went throughout all Syria: and they brought unto him all sick people that