The text of today is known as the Beelzebul controversy. Scribes who come from Jerusalem accuse Jesus of casting out demons by the prince of demons. Jesus refutes their claim by showing how absurd it would be for Satan to cast himself out. The strong man whom Jesus talks about is Satan and the one who binds up the strong man is Jesus himself. Rather than accuse Jesus, the scribes must be able to see that with the coming of Jesus the reign of Satan is at an end.
The sin, which cannot be forgiven, is the sin against the Holy Spirit. Since there is the danger of looking at this sin as a specific sin, Mark clarifies that the reason why Jesus says this is because they accused him of having an unclean spirit. This means that the sin spoken of here is an attitude rather than a specific sin. It refers to the attitude of being closed to the revelation that God is making of himself in Jesus. It is an attitude of closing one’s eyes and refusing to see.
Today the sin against the Holy Spirit is to refuse to believe that the Spirit can transform me. Practically this means to give up even before one can begin. It means to give in or throw in the towel. It means not to give the Spirit a chance to work in our lives. It means a refusal to persevere and keep on keeping on.