I Samuel 16:14 KJV
But The Spirit Of The LORD departed from Saul, and an evil spirit from The LORD troubled him.
King Saul had disobeyed The Lord time after time.
When we live contrary to The Word Of God, which warns us against all evil, we are opening our selves up to the forces of evil.
Ephesians 6:12 KJV
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.
I Timothy 4:1,2 KJV
 Now The Spirit speaketh expressly, that in the latter times some shalldepart from the faith, giving heed to seducing spirits, and doctrines of devils;
 Speaking lies in hypocrisy; having their conscience seared with a hot iron;
Revelation 16:13,14 KJV
 And I saw three unclean spirits like frogs come out of the mouth of the dragon (Satan), and out of the mouth of the beast (The Ant-Christ), and out of the mouth of the false prophet.
 For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
Christ set the demon, possessed and oppressed, free.
Demoniac cured: Mark 1:23-26
Dumb,Blind Demoniac cured: Matthew 12:22,23
Demoniac of Gadara cured: Mark 5:1-21
Dumb Demoniac cured: Matthew 9:32,33
Demoniac girl cured: Mark 7:24-30
Demoniac (lunatic) boy cured: Mark 9:14-39
Demoniac (Deaf and Dumb spirit) boy Cured: Mark 9:14-27
Evil spirits are invited into one's life through disobedience, such as through the practices of, all Occult practices, Divination, Channeling, Astrology, visiting Mediums, partaking of Witchcraft etc.
See The Bible Prayer Line Index: Bible Lessons: Occult.
Demons can be cast out through the power of God, but Jesus warned His disciples....
Matthew 12:43-45 KJV
 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none.
 Then he saith, I will return into my house from whence I came out; and when he is come, he findeth it empty, swept, and garnished.
 Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.
Jesus passed the authority of His Name, on to His followers.
Mark 15:16-18 KJV
 And He said unto them, Go ye into all the world, and preach the gospel to every creature.
 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
 And these signs shall follow them that believe; In My name shall they cast out devils; they shall speak with new tongues; (Acts 2:1-4)
 They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
(Explanation of verse 18)
Acts 28:1-6 :NLT The Apostle Paul
1 Once we were safe on shore, we learned that we were on the island of Malta.
2 The people of the island were very kind to us. It was cold and rainy, so they built a fire on the shore to welcome us.
3 As Paul gathered an armful of sticks and was laying them on the fire, a poisonous snake, driven out by the heat, bit him on the hand.
4 The people of the island saw it hanging from his hand and said to each other, “A murderer, no doubt! Though he escaped the sea, justice will not permit him to live.”
5 But Paul shook off the snake into the fire and was unharmed.
6 The people waited for him to swell up or suddenly drop dead. But when they had waited a long time and saw that he wasn’t harmed, they changed their minds and decided he was a god.
This is not something sought after, but if an evil thing like this should happen to God's people, He will, as He did with the Apostle, deliver them.
Matthew 4:5-7 KJV
 Then the devil taketh Him (Jesus) up into the holy city, and setteth Him on a pinnacle of the temple,
 And saith unto Him, If thou be The Son Of God, cast thyself down: for it is written, He shall give His angels charge concerning thee: and in their hands they shall bear thee up, lest at any time thou dash thy foot against a stone.
 Jesus said unto him, It is written again, Thou shalt not tempt The Lord thy God.
The Power of God, is not for showmanship, or fleshly acts of any kind, religious works included!
Exorcisms, are to be attempted only by Spirit Filled Christian, people, who are given to prayer and fasting.
Matthew 17:19-21 KJV
 Then came the disciples to Jesus apart, and said, Why could not we cast him out?
 And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
 Howbeit this kind goeth not out but by prayer and fasting.
Luke 10:19 KJV
Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you.
Isaiah 58:6 KJV
Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?
Revelation 12:11 KJV
And they overcame him by The Blood Of The Lamb, and by the word of their testimony; and they loved not their lives unto the death.
I John 4:4 KJV
Ye are of God, little children, and have overcome them: because greater is He that is in you, than he that is in the world.