Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see. All the people were astonished and said, "Could this be the Son of David?"
But when the Pharisees heard this, they said, "It is only by Beelzebub, the prince of demons, that this fellow drives out demons."
Jesus knew their thoughts and said to them, "Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? And if I drive out demons by Beelzebub, by whom do your people drive them out? So then, they will be your judges. But if I drive out demons by the Spirit of God, then the kingdom of God has come upon you.
"He who is not with me is against me, and he who does not gather with me scatters. And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. .
Matthew 12 NIV
Apparently Jesus thought the group of Pharisees (a 1st-century Jewish sect) he was talking to had blasphemed against the Holy Spirit. Jesus generally has a rep for being a loving kind of guy, what with sacrificing Himself to save the world from its sins 'n all. That said he sure knew how to dish it out to religious leaders he thought were hypocrites. In this case, the gospel writer tells the story of Jesus giving a blind mute man back his sight and ability to talk.
If you trust the gospel writers, the Pharisees were powerful religious leaders of their time eager to hold on to their standing.
When Jesus apparently healed a man, people wondered if he was the prophesied "Son of David", and it seems the leaders of the religious establishment were a little upset at the attention this young 30-year-old upstart was getting. But instead of offering something better they pulled out one of the oldest tricks in the book of religious ploys, that of accusing your opponent of being in league with the devil. It seems Jesus didn't take kindly to that kind of manipulation.
It's all well and good until Christians sat down and read this book and wondered WTF Jesus meant. Hadn't God promised to cleanse all a person's sins? And if God was omnipotent; if Jesus was all that powerful, why is it that Jesus said there's this one little sin you can't get forgiven for?
Kinda flies in the face of the idea of the all-forgiving God Jesus preached about and claimed to be the incarnation of. As John wrote, "if we confess our sins, he is faithful and just and will forgive us our sins."
Jesus claimed to be working in the power of the Spirit of God, the Holy Spirit, as he healed the blind guy. And the religious leaders accused him of working in the power of the devil.
Jesus essentially said, "Do you know what you're implying? Speaking against me, I can forgive that. But speaking against the work God's Spirit is doing in people's lives..."
Because right throughout the New Testament...and even since the days of Samson and the Spirit of God which "came upon" him, the Spirit of God is described as a force that works within people who trust in Christ to make them better people - to heal them - to cleanse them from their sins. When a person refuses to allow that God's Spirit cleanse them of sin, that person denies God's Spirit entrance to his or her own heart and that person remains unforgiven. Not unlike the first writer to this node described - by his or her own actions a person blocks the means by which God's Spirit cleanses a person of sin. But when a person allows the Spirit to dwell within him or her, any sin against the Spirit has ceased.
Which brings us back to the religious leaders. Apparently they preferred to accuse Jesus of witchcraft - essentially - than to acknowledge that God may have been be at work within him. If they wish to deny God the chance to work in them as well, by rejecting his Spirit as a demon, he may just grant them their wish.
Or perhaps Jesus was just getting cranky after a long day.