What does Matthew 12:30 mean?

Jesus has rebuked the Pharisees (Matthew 12:25–28) for accusing Him of casting out demons by the power of Satan (Matthew 12:22–24). The exact opposite is true. He said in the previous verse, in the form of an analogy, that to accomplish His work on earth, He had to first bind the "strong man," meaning Satan, so He could plunder Satan's domain (Matthew 12:29).

In other words, Jesus came to earth to gather citizens for the kingdom of God by the power of the Holy Spirit (Matthew 12:28). Those who receive Him by faith will be rescued from Satan's domain of darkness and delivered to Jesus' kingdom (Colossians 1:13). Contrary to the Pharisees' charge, Jesus is declaring to them and the listening crowds just how deeply He is working against Satan.

Now Jesus looks the Pharisees in the eyes and draws a line in the sand. Whoever is not with Him in this work He is doing is, by definition, against Him. Whoever does not help Him in His mission to gather citizens into His kingdom is guilty of scattering them. That means that the Pharisees, in their conspiracy to destroy Jesus, have put themselves against the Holy Spirit and the kingdom of God. With Satan, they stand against the work of God.

Jesus makes this absolute statement as a warning to all who hear Him. Nobody can remain neutral on the issue of Jesus (Acts 4:12). He demands full acceptance and participation in His mission from those who are with Him. Anyone who does not join Him is on the other side, no matter how much he or she may wish not to have an opinion (John 3:36).

Mark quotes Jesus as declaring that the positive version of this statement is also true. When His disciples were concerned about someone who was casting out demons in the name of Jesus even though that person wasn't one of them, Jesus told them not to stop the man. "The one who is not against us is for us," Jesus said (Mark 9:40).