The scribes who came down from Jerusalem said, "He is possessed by Beelzebul, and by the prince of demons he casts out the demons." And he called them to him, and said to them in parables, "How can Satan cast out Satan? If a kingdom is divided against itself, that kingdom cannot stand. And if a house is divided against itself, that house will not be able to stand. And if Satan has risen up against himself and is divided, he cannot stand, but is coming to an end. But no one can enter a strong man's house and plunder his goods, unless he first binds the strong man; then indeed he may plunder his house. Truly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin" - for they had said, "He has an unclean spirit."- St. Mark 3:22-30
Those scribes weren’t questioning Christ’s power to exorcise demons. They fully accepted the fact that He had that power. What they got wrong was what they considered the source of that power to be. They were convinced that He had this power because He was in league with the head of all the demons, Satan himself. But they weren’t thinking logically, and Jesus had no difficulty in showing the fallacy in what they were saying.
The essence of exorcism is always that the exorcist calls upon a stronger power in order to drive out the weaker, and this is a basic premise of Catholic exorcism. So when those scribes said that Jesus was using the power of Satan to drive out demons, He makes the powerful point that if there is dissension in a kingdom, then that kingdom won’t last. If there are quarrels in a house, then that household won’t stand for long. So if Satan is making war with his own demons, then Satan would be undermining his own power.
He then goes on to emphasize His point: if you want to rob a strong man, you can’t do it until you subdue him. Once you’ve restrained him, then you can take his belongings. So it should be obvious to the scribes that Jesus certainly isn’t in league with Satan; rather, He was showing that Satan’s defenses were being destroyed. Indeed, someone stronger had arrived, and the conquest of Satan had begun in earnest. Jesus is showing us that there is a struggle between the power of evil and the power of God, and that the power of God will always win because it is definitely not a struggle between equals!
And then there are some very serious words about what has been called “the unforgivable sin.” Now to understand, it’s important to know the circumstances in which our Lord was speaking. When Jesus said, “whoever blasphemes against the Holy Spirit never has forgiveness…” it was when the scribes and Pharisees had claimed that the cures He had done were accomplished, not by the power of God, but by the power of the devil. They were religious leaders looking at at the incarnate love of God, and yet they were able to think that it was the power of Satan.
This, then, is the “unforgivable sin” - to call what is good, evil. To identify the source of good with the devil indicates a moral ruin which will not allow itself to be repaired. When an individual rejects the guidance of God, and loses the ability to recognize goodness when he sees it, having his moral values so reversed that to him evil is good and good is evil, he has made himself immune to being conscious of sin. In such a state, he cannot repent, and if he cannot repent then he cannot be forgiven. That is the “sin against the Holy Spirit,” and is “unforgivable,” not because God cannot forgive, but because such a person will not allow Him to forgive.