"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 12:31-32)
The words of Jesus have distressed some sensitive Christians who fear they may have committed that sin for which there is no forgiveness. We are assured from God's word that there is no sin God will not forgive those who truly repent. Absolutely none. So what is Matthew referring to?
The context of our Lord's words form part of his reply to the Pharisees who disputed the source of his power to work miracles. They never doubted that Jesus was working miracles. However, they attributed his power to his cooperation with the forces of evil, namely Satan. They took this position because they did not want to believe that the miracles confirmed that Jesus is the Son of God, so they had to come up with some explanation as to how he performed the miracles. But Jesus refutes their argument by showing that if he were casting out demons by the power of Satan, as they affirmed, then Satan would be working against himself. Furthermore, Jesus asks, "How can his [Satan's] kingdom stand?" (vs. 26)
Jesus makes clear that he is empowered to work miracles, not by cooperation with demonic forces, but by the power of the Holy Spirit. "But if I drive out demons by the Spirit of God, the kingdom of God has come upon you." (vs. 28) Jesus worked his miracles through the power of the Spirit.
The hardened condition of the Pharisees' hearts is seen in how they attributed demonic power to the miracles wrought by the Holy Spirit. This was not an honest mistake, but a deliberate rejection, a stubborn resistance to the truth. Jesus exposed their argument - that he was in league with Satan - for what it was. Unlike Thomas, who initially refused to believe that Jesus had risen from the dead until he saw the evidence, the Pharisees saw divine evidence pointing to Jesus as the Son of God and rejected it. And, to justify the rightness of their position, they satisfied their conscience by saying that Jesus and Satan were working together. Therefore he could not be from God and they did not have to believe in him.
Jesus said to them that they would never be forgiven, not because God is not gracious, but because they had hardened their own hearts and rejected the truth casting themselves in an impenitent state. The Pharisees had blasphemed against the Holy Spirit and, therefore, for them there is no forgiveness.