Every servant of God must know how to deliver those who are possessed by demons - by exercising the authority there is in the Name of our Lord Jesus Christ. Demons aren't afraid of you or me. They're only afraid of the Lord, Who defeated them on the cross. That's why it's important to know clearly that Jesus Christ took away all of Satan's power on the cross (Col. 2:14,15). This is the good news of the gospel that we must experience first of all and then proclaim to all men. If we believe it, we can deliver others from Satan's power.
We should not be afraid of what demons may try and do to us - because they cannot touch a hair on our heads without God's permission. But many believers are afraid that someone may do some witchcraft on them some day. Why do they have such fears? Because they do not know that Satan was defeated on the cross.
I remember meeting a pastor once who had been sick for a long time, who blamed his sickness on witchcraft that his enemies had done on him. How could that be? Is the Lord less powerful than the power of black magic and witchcraft? No. It was that pastor's unbelief that made him feel that way. No demonic power can stand against the authority and power of our Lord - either on earth or in the heavenlies where the demons operate from (Eph.6:12). If you don't believe that, I'd suggest that you stop serving the Lord and go and do something else. Stop being a preacher because you'll transmit your fear and unbelief to others. Fear is Satan's weapon. Don't let him ever use it on you.
Demons may at times be permitted to harass a believer under the permissive will of God - as in the case of Job. God permitted a messenger of Satan to trouble even the apostle Paul (2 Cor.12:7). That was as irritating to Paul as a thorn in his body. It may have been a sickness or a person who troubled Paul continuously, wherever he went. If we have a thorn in our flesh and we can't pull it out ourselves, we should ask God to remove it. But God may at times say "No", as He said to Paul, if He sees that, that thorn is accomplishing the greater purpose of keeping us humble. Satan was even permitted to hinder Paul from travelling to Thessalonica once. But Timothy could go instead and God's purposes were still fulfilled there (1 Thess.2:18; 3:2).
Let me emphasise this however, that a born-again Christian can never be possessed by a demon. Unfortunately, many preachers these days, are preaching the unScriptural doctrine that believers can be "demonized - and thus bringing many believers under fear and condemnation.
Such preachers cannot quote a single Scripture to justify their teaching. But they say that they've come across such cases in their experience. Thus they exalt their experience above the Word of God. This itself proves that they are wrong.
Christ and a demon can never dwell together in the same heart. Light and darkness cannot co-exist in one place. It is true that some of the Jews in the synagogues where Jesus preached, were demon-possessed. But we don't read of a single case of a born-again believer (after Acts 2), being demon-possessed.
A Christian may be harassed from outside by demons, as Paul and Job were - but that too, only with God's permission. And if God permits such harassment, you can be absolutely sure, as in the cases of Job and Paul, that it will work for your spiritual benefit.
If ever you're in doubt as to whether someone is demon- possessed or not, just ask him to make these three confessions with his whole heart:
1. Jesus Christ is my Lord. 2. Jesus Christ came in the flesh and overcame sin. 3. Satan, you were defeated by the Lord Jesus Christ on the cross. I don't belong to you any more.
Demon-possessed people will not be able to make these three confessions with their spirit.