Jesus said this because they were saying, “He has an unclean spirit in Him.”
With this brief explanation Mark shows for what reason the Lord spoke those severe words. It was the scribes who were calling Christ’s holy and liberating work the activity of an unclean spirit. This accusation was not merely a crude remark. It revealed a deep spiritual rebellion. Here it is very important to notice that Christ gives this warning within a specific situation. He does not want to cast people into pointless fear, but reveals how dreadful it is when a person hardens himself so much that he calls God’s purest work demonic. This should humble all of us and keep us in watchfulness, so that our hearts never become darkened to such a degree.