Can a Christian be demon-possessed?

1. If you're a freewill theist, then that commits you to the proposition that a Christian can be demon-possessed. And that's because a freewill theism believes a Christian can lose his salvation. If, a fortiori, a Christian can lose his salvation, then he can be demonically possessed. It's an argument from the greater to the lesser. If loss of salvation is possible, that's a greater case than possession. If something is possible in the greater case, then that includes the possibility of lesser cases.

Isn't subjection to demons a less radical condition than reversion to an unsaved condition? For a Christian to be possessed would be a horrific degree of subjection, yet lack or loss of salvation isn't a matter of degree but having nothing to counterbalance possession. If a believer can be possessed, that generates enormous tension between two opposing forces, but in the case of lack or loss of salvation, there is no tension because there's nothing on the other side (the Holy Spirit) to counteract the evil. 

Indeed, to be unsaved makes unbelievers liable to demonic possession. Conversion is sometimes an exorcism which liberates unbelievers from possession. So a reversal in your spiritual status, by losing you salvation, would render you vulnerable to the possession all over again. 

Now a freewill theist might object by saying that's not a case of  believer being possessed, but a former believer. But that takes us back to the a fortiori argument. 

2. So the upshot is that I think we have a clearcut answer to the question where freewill theism is concern. The answer is less clear in the case of Calvinism due to ambiguities in the concept of possession, including degrees of sin. Christians are sinners. What degree of sin is compatible with being a sinner? That's one of those sorites paradox questions.