What does Matthew 12:45 mean?

The New Testament unmistakably presents Satan and demons as literally real beings. They are active in the world and do damage to people by taking some level of control over them. Jesus has pictured a scenario where a demon leaves a person behind only to return later and find this "house" cleaned up and put in order. In other words, the person is back to being in his or her right mind and living an orderly life (Matthew 12:43–44).

Now Jesus completes the picture. The demon has found this person to be a good home again, with nothing to protect him or her. The "house" is cleaned, but it's still unoccupied and undefended. Instead of simply moving in, the demon goes out and finds seven other demons—all more evil than it is—to join it in occupying this person. Jesus says the condition of the person is far worse than it was when oppressed by the demon the first time.

Finally, Jesus reveals the point of His story. This "evil" generation of Israelites are in danger of experiencing something similar. Jesus has demonstrated His easy authority over the power of Satan and other demons. He casts them out with a word and has freed many people from demon oppression. Still, this generation of Israelites, as a group, have rejected His teaching and authority over them. They will not receive Him as the Messiah. Jesus warns of what will happen as a result. It's not clear if the warning is of greater demonization in Israel or about what this generation will experience on the day of judgment in the end times.