Here is my take.
Jesus said that when a demon leaves a man, the house is clean. But when/if it returns and finds the clean house, it brings other demons with it to infest the soul worse than before.
In this sense, I do believe nominal Christians can be possess. Moreover, I believe that there are many more demon-possessed people in society than most people realize.
Those in cooperation with the demons appear overall somewhat normal, not exhibiting any social disorder that would put the person at risk of a diagnosis of insane.
It’s the ones who resist the demon’s presence which manifest the bizarre behavior Jesus often encountered. These are actually the ones which are the most hopeful, because Jesus knows their heart. The ones in league with Satan’s minions are the ones which are hopeless.
The key to that parable is that the clean house means that the person has not filled his house with the Holy Spirit so it is subject to repossession.