Jesus gives the perfect defense:
“But if I with the finger of God cast out devils, no doubt the kingdom of God is come upon you.” Luke 11:20
The purpose for Christ casting out demons was to produce belief in Jesus Christ.
Jesus Christ was offering Israel the Kingdom if they believed in Him, but they rejected Him.
Contrary to the conclusions of most of His audience, observant Jews should have seen in His casting out of demons evidence that “the kingdom of God had come upon you” (them).
Never once in the Old Testament are we told of demon possession or of any one casting out demons.
Moreover the book of Revelation (particularly chapters 6-19) which describes the seven years of tribulation does not give one single record of people being demon possessed or the casting out of demons. And, yet, we know that demonic activity will run ramped during the seven years of tribulation on earth.
Does demon possession occur?
Of course it does.
Demon possession occurred and will occur from the point of mans disobedience in the Garden of Eden until the last person born on earth (Demonic activity will probably not occur during the millennium, because Satan will be bound for 1000 years while Jesus Christ rules and reigns on the earth). Demon possession and the casting out of demons will probably not happen during Christ’s Kingdom on earth – the Millennium.
Demon possession and the casting out of demons occurred right through the Old Testament time and through the Church Age that we live in today.
No doubt demonic activity and the casting out of demons will be at its peak during the seven years of tribulation. God the Holy Spirit will not be on earth to restrain the evil one (Satan) during the tribulation and Satan will have free reign during the tribulation.
So what was the purpose of recording demonic activity in the Gospels and the book of Acts?
The purpose of emphasizing and recording demonic activity and the casting out of demons during the time of Jesus Christ and the 12 apostles was to authenticate the person and the message.
That is, it was to show that Jesus Christ is the Son of God who was sent from God and His message is God’s message, and the 12 apostles were sent from God and their message is God’s message. Therefore listen to Jesus Christ and the 12 apostles.
Demonic activity has existed and occurred throughout human history, but the only time it was recorded and emphasized in the Bible is in the Gospels and Acts; that is, during the time of Jesus Christ and the 12 apostles.
Again, as horrible and as demonic as the seven years of tribulation will be, the book of Revelation does not record one single incident where a person is demon possessed or a person casts out a demons. The tribulation will be loaded with demonic activity but the Bible does not record individuals being demon possessed or individuals casting out demons during that period of time.
Who can be demon possessed?
Only unbelievers in Christ can be demon possessed. Believers in Christ cannot be demon possessed. “Ye are of God, little children, and have overcome them: because greater is He (God the Holy Spirit) that is in you, than he that is in the world” (1 John 4:4).
Who can cast out demons?
Believers in Jesus Christ have the power of God within them (that is, God the Holy Spirit) to cast out demons.
Interestingly, unbelievers can cast out demons also, but only by the power of Satan. Please look at Acts 13:6-13.
The purpose of Paul’s confrontation with Bar Jesus in Acts 13:6-13 was to show that Bar Jesus who was casting out demons was not a believer in Christ and neither he nor his message was from God. Bar Jesus was a false prophet casting out demons by the power of Satan.
Living the Christian life and casting out demons is a spiritual battle. This is what Paul said in Ephesians chapter 6:10-24:
 Finally, my brethren, be strong in the Lord, and in the power of his might.
 Put on the whole armour of God, that ye may be able to stand against the wiles of the devil.
 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.
 Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand.
 Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness;
 And your feet shod with the preparation of the gospel of peace;
 Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked.
 And take the helmet of salvation, and the sword of the Spirit, which is the word of God:
 Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and supplication for all saints;
 And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,
 For which I am an ambassador in bonds: that therein I may speak boldly, as I ought to speak.
 But that ye also may know my affairs, and how I do, Tychicus, a beloved brother and faithful minister in the Lord, shall make known to you all things:
 Whom I have sent unto you for the same purpose, that ye might know our affairs, and that he might comfort your hearts.
 Peace be to the brethren, and love with faith, from God the Father and the Lord Jesus Christ.
 Grace be with all them that love our Lord Jesus Christ in sincerity. Amen.
The power of God is in the Gospel of Jesus Christ.
“For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to everyone that believeth; to the Jew first, and also to the Greek.” Romans 1:16
What is the Gospel? (1 Corinthians 15:1-4)
 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
 And that he was buried, and that he rose again the third day according to the scriptures:
The Gospel is the death, burial, and resurrection of the Lord Jesus Christ (1 Corinthians 15:1-4).
Finally, what is God’s message for us? John 3:16-18:
 For God so loved the world, that He gave His only begotten Son, that whosoever believeth in Him should not perish, but have everlasting life.
 For God sent not His Son into the world to condemn the world; but that the world through Him might be saved.
 He that believeth on Him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.