I’m definitely not an expert, but I believe demon possession comes in different ways. One involving free-will, is when a person chooses to repeatedly commit a sin, such as pornography, there comes a time where they no longer have control over their choice and possibly a demon controls them in this area. This wouldn’t be full demon possession, as they probably have many other areas still under their control, but a door has been opened for a demon, and it will try to invade other areas of their life.
A related question would be, do Christians ever have areas of their lives where demons are in control of their free-will.
I believe a biblical example of a Christian having a demon is the Apostle Pauls “thorn in the flesh”, which was a “messenger of satan”, which Paul prayed 3 time that it would “leave him”.
Your question is a good one, and I’m probably not the one to answer it to your satisfaction. But for what it’s worth: I doubt if anyone consciously chooses to become demon possessed. But, when we choose to sin over and over without repenting, we become in bondage to that particular sin (slaves to sin). Sometimes, we think we can get away with sinning a particular sin, and we can quit whenever we want to, but each time we commit the sin, it gets a stronger hold on us, and it takes a merciful act of God to free us again.
Although I believe there are demons, I don’t like to give them too much “glory” by mentioning them. I prefer to call them the “enemy”. Also, I don’t have any trouble with thinking the things that control us are our own weaknesses as humans, such as negative emotions, desires, etc.
I certainly do believe that can happen. But I tend to think the majority of cases are more like what Lynda is talking about. (And good point to her on Paul’s “thorn in the flesh” btw!)
Another category would be where demons are simply attacking someone and causing effects along the way. This would be a more extreme version of the temptation any of us suffer (insofar as demons are involved in those temptations).
To what extent are persons personally responsible for the existence of the condition and/or the continuing results? That’s for God to sift, I think, not us. But to give a paradigm of principle: on one hand we have cases like, say, what Christ was being accused of (deliberate cooperation with Satan), where the cooperation would be analogous to our cooperation with God. At the other end of the paradigm we would have things like Luther’s famous description of depression as being “mugged by Satan”. Closer to the latter side would be cases where the demons have succeeded in twisting the personality until the person willingly joins in the sinning out of a damaged psyche or simply because the pressure (whether in pleasure or pain but especially the former) is too strong to resist. (A succubus attack would usually fall into that category, for example.) Closer to the former side would be cases where (as Lynda was describing) someone gets into the habit of sinning persistently on his or her own initiative, and then demons take advantage of the situation by reinforcing and riding the condition. (This appears to be behind some of the demonization cases cured and warned against by Christ in the Gospels.)
None of which is to deny that (quasi-)temptations and mental disorders can be instigated by natural causation, too. The stories of succubi could be explained almost if not altogether entirely by a psycho-biological problem men routinely suffer from, for example. But then, instigation by natural causation is not itself necessarily exclusive to reinforcement of the effect by rebel supernatural agency. There is a long-running tradition throughout the history of world religions (including Judeo-Christianity), that pictures demons finding a naturally occuring chaotic situation and making it worse than it otherwise would be. Tornadic activity features some very disturbing characteristics along this line (and has also long been associated with demonic tampering. Note that the calming of the storm on Galilee by Christ is literally concerned with exorcising a “whirlwind” that had descended upon the lake.) One of the more interesting things I’ve read on the topic involves parallels between tornadic energy-reinforcement behaviors and zero-point energy experiments.