This transgression is commonly called “the unpardonable sin,” something so grave that it will not be forgiven, either in the present age or in the next one, even though God is normally eager to forgive.
Blasphemy is not talked about much these days, since our culture cares little about the things of God. The word “blasphemy” comes from two roots that together mean “injurious speaking.” Granted, speaking (or writing) that causes injury is quite common these days, but blasphemy belongs in a separate category because it has God or something sacred as its target. Thus, blasphemy is “a dishonoring of God or sacred things,” whether done deliberately or not.
Jesus' words in Matthew 12 are a strong enough warning by themselves, but the parallel account in Mark 3:29 makes the consequences of this even more plain: “He who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation” (emphasis added).
The wider context of these verses helps us to understand this stark warning, beginning in Matthew 12:22 with Christ healing a demon-possessed blind-mute. Because of the Pharisees' hard hearts, they would not accept that this had been done through the power of the Holy Spirit, so they tried to diminish this work of God by claiming it was performed by the power of Satan.
In verse 33, Jesus says to evaluate a matter based on the fruit that is produced. The Pharisees should have been able to see the supremely positive fruit that He was producing, and at the same time, He was pointing out that the fruit they were producing was rotten. In verses 34-35, their speaking evil against the power of God reflected the evil in their own hearts. While the Pharisees belittled the miracle that had just taken place, Jesus says in verse 36 that even idle or careless words must be accounted for in the Day of Judgment. Verse 37 warns that our words will either justify us or condemn us, putting the Pharisees on thin ice.
Notice, though, that He does not state directly that these Pharisees had committed the unpardonable sin. They did commit blasphemy serious enough to evoke a thunderous warning, but it appears that Jesus may have made some allowance for the Pharisees because, in His taking on the form of a bondservant (Philippians 2:7), they were confused about who He was. His true identity as the Son of God had not been revealed to them (as it had been to the disciples; Matthew 16:16-17), so He declared that they could be forgiven the blasphemous things they said about Him. He did not mean that blasphemy or other sins are no big deal, but rather that it is possible for those things to be forgiven upon repentance, in contrast to a transgression that cannot be forgiven at all.
Remember, the Pharisees triggered this warning by attributing the outworking of God to the Lord of Flies (Beelzebub). It included a rejection of God's nature, power, and activity. The conversation between Christ and Nicodemus shows that some of the Pharisees would acknowledge that Jesus was a Teacher sent by God (John 3:1-2). Yet, Matthew 12:14 states that these Pharisees were plotting against Him, so they had malicious intent.
Even so, a measure of ignorance remained. Paul says in I Corinthians 2:8 that if the rulers of the age—which would include the Pharisees—had full comprehension, they would not have crucified the Lord of Glory. Despite their attitudes approaching the point where they would be unable to repent, their lack of full comprehension of who they were opposing meant that repentance could still be possible once their eyes are opened. Due to their ignorance, they were not guilty of conscious rejection of the Spirit of the Most High God.