What does Matthew 12:28 mean?

Jesus has shown that the Pharisees' slander—that He casts out demons by Satan's power—is ridiculous (Matthew 12:22–24). First, it would mean Satan is fighting a battle against himself. Second, it would call into question by whose power the Pharisees attempted their own exorcisms (Matthew 12:25–27).

Now Jesus adds that the Pharisees are missing the most important point of all. If He is truly casting out demons by the power of God's Holy Spirit, then the kingdom of God has come, at last. Of course, this is exactly the conclusion the Pharisees are trying to avoid. The problem here is not merely that they "do not" believe. It's that they "do not want to" believe that Jesus is the Messiah.

Christ leaves His critics no room to escape the most rational conclusion before them. He does indeed cast out demons by the Spirit of God. The Jews widely believed that God's Spirit had left Israel after the prophets had died out hundreds of years earlier. If the Spirit was back and working through Jesus, as it obviously was, the time of the Messiah has come. More than that, the King has come, and the kingdom of God has been initiated, even if it was not fully established.

Jesus' ability to so quickly and easily cast out demons was evidence that He was the Messiah and King. Those who made a conscious effort to resist those truths were in a dire spiritual condition.