Hi ya Bill,
When I read your post I started racking my brains, thinking and asking myself, 'where is there an example in the Bible of anyone who was demon possessed and a man of God (Old Testament) or a Christian (New Testament)?' Closest I came to was King Saul.
1 Samuel 19:23, not exactly convincing evidence, that was just a temporary state of mind. Immediately afterwards the Holy Spirit exited Saul and the murdererous demon resumed control of his head. Maybe there are better examples that I've overlooked?
In the Gospels and Acts there are many accounts of poor unfortunates who are demon possessed coming into conflict with Jesus and and the gospel. Top priority for either Jesus or the evangelist is to rid the person in question of his or her demon possession. There are no examples of this being a gradual process, the Holy Spirit cannot and will not share lodgings with a demon. The unbeliever has a binary choice to make, you either accept Jesus or reject him and Jesus himself said that if you don't accept him then you have rejected him. Your faith cannot be your pasttime or your hobby, no it has to be your imperative, the reason you get up in the morning, God wants to be your everything.
That's not to say there won't be slip-ups, there won't be times when you backslide or sometimes fall off the wagon altogether, like the prodigal son. But that's not demon possession, if a demon is in control of your head then you are not a Christian. I suggested to the OP that Satan can try to deceive a Christian that he's in control, but he isn't, don't fall for it. What is more likely is that he may have a mental health issue and that Satan is exploiting that to try to convince him that he's possessed. From his post, Satan clearly isn't in control and more likely he just needs to get some therapy.
So in summary, yes I slip up, yes I sin, probably more than pretty much everyone in this forum but that doesn't mean to say I am possessed by a demon. No it means that I am filled with the Holy Spirit as per Jesus's promise to me but every now and again there's a bump in the road, as serious as that. Thank goodness that every single one of those sins are covered by the death of Jesus. Thank you Jesus, thank you God.