First, it is important to note that facts or truths exist apart from the Bible. For example, gravity is a scientific fact, which (to my knowledge) the Bible does not specifically comment, but it’s a fact, nonetheless. Likewise, that Alexander the Great existed is a historical fact to which the Bible does not specifically refer, but his existence is a fact. So, facts are facts and truth is truth whether or not the Bible comments on them.
There are biblical doctrines/teachings (those which are stated in the Bible). And then, there are non-biblical doctrines/teachings; those not stated in the Bible. But non-biblical doctrines are not necessarily wrong if they are not inconsistent or contradictory to the Bible and if they are factual (e.g., gravity, Alexander the Great, etc.). Non-biblical scientific, historical, or clinical facts or truths are not equivalent to the Bible, the Word of God, in that they are not inspired Scripture, but they are facts or truth, nonetheless.
Second, there is absolutely no biblical proof to say that Christians cannot be demon-possessed. And while I agree that there is no specific verse in Scripture that states that a Christian was demon-possessed (in those exact words), that does not mean or conclude that Christians neither cannot be nor were not. Silence does not establish biblical doctrine.
Opinions based upon incomplete or faulty information do not compare at all to the validity of teaching based upon clinical facts or studied experiences which are in no way inconsistent with or contradictory to the Bible. Experience is not an invalid method of establishing facts or truth if the experience is not inconsistent with or contradictory to Holy Scripture. For example, according to God’s law, a person could be executed on the basis of two eye-witnesses (their personal observation of an experience) who viewed the guilty person’s offense (Heb. 10:28), such as blasphemy (John 10:33). Life or death rested upon the experience of two witnesses of a certain event. If experience was not a valid criterion for establishing the truth or facts, then God would not use that method for such a serious matter as the termination of a precious life. Obviously, people’s experiences or observations of events did determine truth or facts and was used in serious matters.
Even Dr. Luke, the author of the Gospel According to Luke, uses people’s eye-witness experiences and his investigation of them to establish truth (Lk. 1:1-4). Luke’s investigation of these people’s experiences and observations of events did determine Holy Scripture as God superintended upon there being recorded.
Third, God and Satan/demons can and do co-exist side-by-side in the universe. Because God is omnipresent (Psa. 139:7-8; Jer. 23:23-24), He, therefore, exists side-by-side wherever Satan and every demon exist. Specific examples are found in 1 Kings 22:20-23; Job 1:6; 2:1; Rev. 14:9-10 with Rev. 20:10; 12:10; Matt. 4:3, 5, 8. Since God and Satan/demons can and do exist side-by-side, it is therefore, not impossible for them to both live inside the body of a Christian/believer based on the incompatibility of proximity theory. Demon-possession of a Christian, however, does not imply ownership by the demon (because God owns the believer – 1 Cor. 6:20), but it does imply control of the believer to some degree in certain areas of his/her life. And yes, believers/Christians can be controlled by Satan, as seen in Acts 4:32; 5:3. Or, as seen in 2 Tim. 2:24-26, where those who are held captive by the devil to do his will are Christians (Dr. J. Walvoord’s The Bible Knowledge Commentary, N.T., p. 755; J. D. Kelly’s A Commentary on the Pastoral Epistles, p. 190).
Fourth, reasoning and logic are not evil, but are God-given abilities. In fact, God encourages man’s use of reasoning in Isa. 1:18. Jesus uses logic in Matt. 11:2-5 and 12:24-29. The apostle Paul uses logic and reasoning in Acts 17:2-3, 17-31; 18:4, 19 and Acts 19:8-9. Peter uses logic and reasoning in Acts 11:2-18. Though man’s logic and reasoning can be inaccurate because of his sinfulness and spiritual blindness or mental abnormalities, logic can also be used correctly and understandably as a valid basis for establishing or conveying truth or facts, as has been shown in the above passages.
Fifth, Dr. M. Unger changed his view (from Christians not being able to be demon-possessed to the fact that they can be) because clinical case studies on numerous occasions by numerous godly men showed that Christians were demon-possessed and because he found no biblical basis to show that Christians could not be demon-possessed.
Sixth, Scripture (the Bible) is sufficient to derive “biblical” doctrine. But as has been shown earlier, there are facts, truths, doctrines/teachings apart from the Bible that are equally correct or valid that can be derived from science, history, carefully investigated experiences or observations, such as in Lk. 1:1-4, in gravity, in the existence of Alexander the Great, etc. The latter are not called “biblical” doctrines, but they are factual/true, nonetheless.
Seventh, what may seem logical to you or others (regarding disbelief in Christians being demon-possessed because of the absence of the words “demon-possessed” or the “casting out” of them in the Epistles) may, in fact, not be the truth. For example, just because the words “trinity”, “rapture”, or “pre-millennialism” do not appear in the Epistles, does not mean that the concepts are not taught either there or somewhere else in the Bible. Likewise, just because new Christians are nowhere commanded to be water baptized in the Epistles, does not mean that they shouldn’t be. Or, because Christians are nowhere commanded to confess their sins to God to remain in fellowship with God, does not mean that we shouldn’t do so. Or, because nowhere in the Epistles are Christians commanded to make disciple, does it mean that they shouldn’t? If these doctrines are nowhere to be found in the Epistles as commands for Christians to obey, then why do nearly all Christians teach them? Why, because Jesus commanded His disciples to make disciples to all nations as they were going and baptizing, and then to teach these new believers all that He had commanded them (one of which was to make disciples – Matt. 28:19-20), and because we see it being done by Christians in the book of Acts in fulfillment of this command in Matthew. So, it’s not necessary to repeat in the Epistles. The same is true of baptism. The command is in Acts 2:38 and 22:16, and Christians were doing it. Therefore, it is unnecessary to repeat in the Epistles. But it obviously was being taught to all the new converts in all the new places that Christianity spread; otherwise, they wouldn’t know to do it. However, we have no record of it being commanded in the Epistles. Does that mean it wasn’t being done or shouldn’t or couldn’t do it? No! It was done (1 Cor. 1:14-16). Likewise, with confession of sins by Christians. Though there’s no command nor example of anyone doing it in the Epistles (confession of sins to God), we assume it was being done and that we Christians should do it too. Why, because of “logical” deduction based on 1 John 1:9. Apparently, it was being taught and done though there is no record of it in the Epistles.
So it could and probably was with the issue of demon-possessed Christians and the casting out of demons from them. Demon-possession and casting out demons from people was common place in the Gospels and Acts (e.g., Matt. 8:16, 28, 32; 9:32-33; 10:1; 12:22; 15:22; 17:14-18; Mk. 1:32, 34; 5:15; Lk. 4:33-35, 41; 10:1, 17, 20; 22:3; Acts 16:16, 18; 19:12-16). These demons were universally cast out by Christ’s authority, whether given to people by Christ in pre-Church times or claimed by believers in the Church era. And the manner of casting them out seems to be almost always by verbal rebuke in Christ’s authority/name and faith in such authority. Although these passages refer (as far as can be deduced) to non-believers being demon-possessed, it does not mean that believers weren’t or could not be (any more than believers weren’t discipling or confessing their sins in the Epistles). And because careful, clinical investigation of hundreds of Christians who were or are definitely demon-possessed (while being a Christian) has been conducted, reason/logic tells us that Christians can, in fact, be demon-possessed (Dr. M. Unger’s Demons in the World Today; N. Anderson’s The Bondage Breaker; Dr. F. Dickason’s Demon-Possession and the Christian; Dr. K. Koch’s Occult Bondage and Deliverance). The same authority (in Christ) and method of casting out demons from non-Christians could/should be used with Christians, as the Bible doesn’t record any change of instructions for use with demon-possessed Christians. So, it must have been the same, even as instructions for water baptism aren’t mentioned in the Epistles because they apparently followed the Gospel/Acts models of baptizing.
Charismatic experiences (e.g., tongues, prophecy, knowledge) are wrong because the Bible specifically states that these things have ceased (1 Cor. 13:8-10). Catholic tradition is wrong if it contradicts the Bible or other established facts. Extra-biblical facts or truths are still facts or truths whether or not they are contained in the Bible, but they are not equal in the sense that they are not the inspired Word of God as is Scripture.
Eighth, casting out demons is never mentioned as a gift of God. Never is it mentioned in any of the lists of spiritual gifts (1 Cor. 12; Rom. 12; Eph. 4), whereas “healings” is mentioned as a gift. Casting out demons is an authority Christians have in Christ, as already shown.
Ninth, believing that Christians can be demon-possessed is not necessarily a new movement. I’m not aware of any historical documentation that can support that view. A person could say that the pre-tribulation, pre-millennial view is a new movement too because the Church popularized other views throughout the centuries. But that doesn’t mean that pre-tribulation, pre-millennialism is wrong (not biblical) or that some Christians didn’t believe it throughout Church history. Believing that Christians can be demon-possessed in no way distracts us from the Word of God nor does it reduce our fidelity toward it.
Tenth, experiences of success in casting out demons should not convince us that it is from God if the Bible stated that it wasn’t so, or if it was done in an unbiblical manner. But since the Bible does not negate such experiences, then facts are facts and all the more convincing when done by numerous godly men on numerous occasions (who are true to God’s Word and living obediently to Him) and investigated carefully.
In conclusion, since Christians being demon-possessed is not spoken against in Scripture, and since God and Satan exist side-by-side in the universe already, and since hundreds of clinical cases of demon-possessed Christians have been documented; therefore, Christians can be demon-possessed and have been.