Demon possession, obsession, and oppression...
Demon oppression is when a demon spirit makes an attack on the physical person. Jesus went about healing all who were oppressed of the devil.
"How God anointed Jesus of Nazareth, with the Holy Ghost, and with power, Who went about doing good, and healing all that were oppressed of the devil; for God was with Him."
Jesus recognized that some sickness and disease was sourced from the devil and his demons. Demon oppression can come to saint and sinner alike, and a rebuke of the devil and demons that oppressed such a one, is needed to return health to them.
Demon obsession is when the demon plague the mind and emotions of one, suggesting evil, blinding from good, hiding from the one obsessed the truth. False accusations, evil speaking, fear and torment come from the person who is demon obsessed. This can happen to saint or sinner.
Demon possession, can only happen to the unsaved, for the saved have Jesus dwelling within them, and He will not share the room with the devils. The demons are disembodied, and missing the body they once had, they seek to control the body of another. When unable to control the man of the Gadarenes, they went into the pigs, who chose suicide to having demons possess them...
Demons when they possess a person, take over the will. The saints can, in the name of Jesus, cast out the demon, to give the will of the person another chance to choose Jesus. If they do not choose Jesus, many more demons will return later, to move back into the person. The exorcism is possibly temporary therefore.
So there is demon possession, demon obsession, and demon oppression.
Demons are subject to the Lord, and to us, who use the authority given to us by the Lord.
Matthew 8:16-17 (KJV)
16 When the even was come, they brought unto him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick:
17 That it might be fulfilled which was spoken by Esaias the prophet, saying, Himself took our infirmities, and bare our sicknesses.
Matthew 12:28 (KJV)
28 But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you.
Mark 16:17 (KJV)
17 And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
Luke 10:17 (KJV)
17 And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name.