you are speaking of possibilities not the word of God. Jesus said not me Light has no fellowship with darkness. Jesus also said that a kingdom divided against itself cannot stand. You are using human reasoning and speculation to suggest that a Christian willfully sins can also be demon-possessed?
Human reasoning? You are the one who cannot show me where the Bible says a Christian cannot be demonized, but wants to make a solid doctrine about what is problem. 'Demon possessed' comes KJV 'possessed of..." whatever... an unclean spirit or however those verses are worded. The word 'demonized' is taken from the Greek. Our -ize ending comes from Greek anyway. Some translations make it sound like the demon owns the person. And so some people come up with 'demon oppressed' as opposed to possessed. But the original language didn't have this 'possession' in the form of ownership.
Paul wrote light has no fellowship with darkness when he told believers not to be unequally yoked with unbelievers. But is it possible for a believer to be unwise and disobey? Don't you think so? If a Christian marries an unbeliever, does that mean he or she was never a real Christian to begin with... or does that mean he or she made a mistake. John said if we walk in the light as he is in the light, we have fellowship with one another and the blood of Jesus Christ cleanses us from all sin. But what about people who go off into unrepentant unconfessed sin and get cut off from fellowship?
Here is the problem with all this
real life. In real life, if you go into a 'deliverance ministry' scenario and the person voice gets all weird and they have difficulty saying the name of Jesus and say a demon torments him, but he believes in Jesus, confesses his faith in Jesus, his prayers seem sincere, he cares about Biblical things-- honoring his father, prayer, etc. He's been baptized. His faith seems genuine and if the weird stuff didn't happen and you didn't hear about him being demonized, you'd think he's a Christian. So what do you say? Do you tell him his faith was not genuine, that it didn't really take? Is his baptism invalid and needs to be repeated? Are you going to judge his salvation and say he is not a Christian?
When you are in one of these scenarios with someone who by all accounts is supposed to be a believer, who says 'No!' in a little bit deep voice when they tell the spirit to come out, do you say she is not a Christian. Do you have that right.
The problem is the variety of stuff that happens in real life, and the doctrine that says a believer cannot be demonized is based on some loose reasoning using scriptures that directly address the issue.
Please show me in the word of God where a person was passed by a demon and a Christian. Not influenced, tempted, oppressed, but
possessed by a demon. The sin that Christians do is willful
Show me your name in the Bible where it says you exist. Show me where it says automobiles will fly. "It's not in the Bible, so it cannot happen." is not a reasonable argument because we both know the Bible does not predict everything. There isn't a verse in the Bible that says that no Christian can be demonized.
Where are your categories in the Bible? I know it says that the Devil tempted Jesus. Does it say demons tempt people? Where in the Bible is 'demon oppressed.' I haven't found that. The translation said 'possessed' and that sounds like ownership. That's probably why so many Christians say that true Christians can't be demon-possessed, because it sounds like ownership.
The closest scripture I can think of to this is Satan filling Ananias heart. Either Satan got in that heart himself, or he put stuff in it. If the former, well that's pretty close to demonization except I do not know that Satan is the same class of spiritual entity that demons are in. The Shedim and sons of God may be different types of spirits in Deuteronomy 32 (using Dead Sea Scrolls which seems to align with the LXX for verse 8.)
I learned in Sunday school that you tell the demon to go in Jesus' name and it goes. We had this song, "In the name of Jesus... demons will have to flee...." So I thought they came out in a split second. A Christian author I know said that in the Christian tradition it may have to be repeated many times and referred to a grammatical form used in a passage where Christ cast out a demon. With the Gedarene demoniac, Jesus had told the spirit to come out before He found out that it was legion.
In real life things can be different from what 'experts' who do not know what they are talking about think. I do not claim to be an expert, but I saw a young woman saying 'no' in one of those scenarios and people who knew her told me she was a Christian, etc. I was an observer of a brief part of it, and this happened when I was quite young. I also realized from that and other experiences that exorcisms do not always happen in a split second.
I do not think Christians walking holy with God need to be scared of getting demonized out of the blue. But Jesus told his own disciples if each of them did not forgive their brother from their heart, that the same would happen to them... after telling them about the unforgiving servant being delivered over to the tormenters until he should pay the debt. If 'tormenters' means or includes demons, this may be a relevant passage.
There are also people who profess faith in Christ, get baptized, and start getting discipled who might go into some kind of sin. If they are doing witchcraft, divination, etc. or idolatry, I could see how those could be closely connected to demonic activity. If someone is under church discipline and has been delivered over to Satan for the destruction of the flesh that his spirit might be saved in the day of the Lord, don't you think that that person might have some kind of problems with Satan?
That is why James chapter 1 which is written to Christians not the unsaved said:
12 Blessed is the man who endures temptation; for when he has been approved, he will receive the crown of life which the Lord has promised to those who love Him. 14 But each one is tempted when he is drawn away by his own desires and enticed. 15 Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.
I do not see Demon mentioned.
How is that relevant to what I wrote? You are the one mentioning demons doing tempting. I suspect they do that, but I do not know of any scriptures that specify that demons tempt except passages Satan or 'the Devil'. Even if Satan is involved, lusts are involved also when one succumbs to a temptation.
But I see yet another verse that does not say a Christian cannot be demonized in some cases.
16 Do not be deceived, my beloved brethren. 17 Every good gift and every perfect gift is from above, and comes down from the Father of lights, with whom there is no variation or shadow of turning. 18 Of His own will He brought us forth by the word of truth, that we might be a kind of firstfruits of His creatures.
How do you think this addresses the issue? Does this mean Satan could not fill Ananias heart, or that a man in the church could not be delivered over to Satan for the destruction of the flesh?