What does Jesus mean when he talks about evil spirits? Does he mean literal demons? Today, those who believe in demonic possession are very much in the minority (at least in the west). Is he speaking metaphorically? It doesn’t seem so:
When an evil spirit is driven out of a man, it walks through dry places, seeking rest, and finds none. Then it says, “I will return to the house from which I came.” When it comes, it finds it empty, swept, and garnished. Then it brings seven other spirits, each more wicked than itself, and they enter in and dwell there. In the end, the man is worse off than he was at first. So it will be for this wicked generation.
In Jesus’s time, many psychological problems were attributed to demons, and curing mental problems by faith was seen as casting them out. If Jesus didn’t actually believe demons were their cause, he would still have spoken about mental problems in a way people would understand. What he said makes very good sense, if I paraphrase it:
When obsessive desires are driven out of man, the old habitual behaviors will eventually return. If, when they do, nothing has taken their place, their denial will have made them even stronger than they were, and they will reassert themselves. In this case, the man will be worse off than he was at first.
Think about dieting. If you use pure willpower to resist temptation, you can succeed for a while, but eventually, your will will crumble, and when it does, very often, you will gain back all the weight you lost and more. What Jesus is saying is that you need something more that willpower (or momentary faith) to remain free of your desires. You need mental certainty that you are on the right track and the spiritual strength to persevere. If you stumble, you need to get back up and get back on the path. For Jesus, that path is the one that leads to the kingdom of heaven.