Hi. Having been involved in deliverance for 24 of my 29 yrs via our church ministry I've seen quite a few engagements in the supernatural.
To answer some things that have been said here:
1. We need not fear the devil...but fear can come and curses can come if we involve ourselves with idol worship or any act of divination; and yes, devils can harrass and even oppress Christians until they confess such involvement.
By involvement you mean by actually doing the actions. You are not saying someone see's a rock and thinks its pretty and has a pile of crystals sitting in the door way as decorations. Your saying christians that actually practice or attempt to try these acts of divination.
2. Christ has secured our eternal salvation and the defeat of Satan was made by His shed blood and death on the cross but again, if Christians practice sin they will feel the sting of sin and Satan sooner or later. They do not become 'possessed' but they can become 'oppressed', which looks much like the same thing.
Of course but I think that is something very different then what we are talking about now. If a man cheats on his wife and he has nightmares, he becomes addicited to the other woman then that is a direct reaction to his sin...............But the devil trys to oppress christians in other ways are you familar with Job?
3. The battle is not just without...it is within.
II Corinth. 10:4-5
4 (For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds
5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
So the battle is not always outside of us...it can and often is inside depending on our sinful habits, attitude, or desires.
As always, the Word of God, faith in Christ, and the shed blood of Jesus is sufficient to defeat the enemy and force him to leave us.