Yes, scripture does mention an unforgivable sin. It is referenced in Mark 3:29 and Luke 12:10. Each passage says, in their own way, that anyone speaking against the Son of Man will be forgiven, but anyone speaking against or blaspheming the Holy Spirit will not be forgiven.
There are various schools of thought on this. When I first left the Pentecostal Movement because I could no longer support the “Evidence of Tongues,” I tried to explain it to a Pentecostal relative. She refused to even listen to my reasons because, in her words, I was in danger of blaspheming the Spirit. Of course this was a bad interpretation often used by signs and wonders churches to admonish those calling attention to their unscriptural teachings—“The Holy Spirit taught me this, so if you disagree you are blaspheming the Holy Spirit!”
The fact that the passage mentions an unforgivable sin without further defining exactly what that entails leaves it open for abuse and causes problems with those who fear accidental transgression will leave them without hope of repentance. Some will even use Hebrews 12:15-17 to expand the danger out to many areas of life. It speaks of Esau selling his birthright for a pot of beans and then finishes by saying, “For you know that also afterwards, when he wanted to inherit the blessing, he was rejected, because he did not find an occasion for repentance, although he sought it with tears.” It is claimed that “he did not find an occasion for repentance although he sought it…” means that he was unable to be forgiven. Actually, it means he tearfully sought to repent of the sale of his birthright and regain it, but was not able to do so—the sale had been final and God enforced it as such. After all, God had even prophesied that the older (Esau) would serve the younger (Jacob) before the two were even born.
So what exactly does Jesus mean by this reference? To be honest absolute certainty is not possible since he does not himself define it. However, this is not what causes the confusion. The problem is that for so long the passage has been taken from its context and read separately. The warning about the blasphemy of the Holy Spirit must be read in its proper context.
Luke simply lists this teaching in with other teachings and doesn’t really give a context to help understand. Remember each gospel writer had a purpose for their writing and in keeping with acceptable practice for historiography of the time they could leave out or focus on details, depending upon the purpose of their writing. This was not a time of scholarly history, but was a time of purposeful historical writing—compare Suetonius to other ancient historians for such an example. This does not undermine inspiration. It shows God was inspired them to write history of their day, in their fashion, and not history in our modern scholarly fashion.
Mark, fortunately, gives more detail on the context of this warning. Jesus had been performing many miracles to demonstrate his divinity and his role as the Messiah. The Scribes (Mark 3:22) accused him of doing these miracles, not by the power of the Holy Spirit, but by the power of Beelzebul (Baal zebul—Lord of flies). This name had been adopted from the Canaanite pantheon as synonymous with Satan. They were claiming the wonders worked by the Spirit in the name of God were actually being counterfeited by Satan. They claimed Jesus was a tool of Satan. This is why Jesus said (my paraphrase), “Say what you want about me, but there is no forgiveness for those who attribute these acts of the Holy Spirit to Satan.” Mark even tells us (3:30) this was why Jesus said it.
So this leads to two questions:
One, was he saying the scribes had committed this sin and could never be forgiven?
Two, is this a danger we have to worry about today?
I find it hard to interpret this to mean that Jesus is saying the scribes could never be forgiven for what they just said. He is warning them about their actions and the dangers of that action. If they were already lost then the warning would be meaningless. The thing to remember is that these wonders Jesus was performing all conformed to what the Jews had always believed the Messiah alone would be able to do. They already believed the Messiah would raise the dead, heal the blind, cleanse lepers, forgive sins, etc. Now they see one doing these very things and, rather than believe, they attribute them to Satan. The reason they cannot be forgiven is because so long as they continue this attribution they will be unable to recognize the only forgiveness available—forgiveness in Christ. The word for blaspheme in this case is in the aorist tense. According to Dana & Mantey, the Greek aorist has temporal significance only in the indicative mood. But here is it in the subjunctive. Subjunctive means “if you …” This tells me his warning of something that has not yet happened. The aorist in this case also tells us it is not just saying, “if you do this you will not be forgiven,” but instead means, “If you blaspheme the Holy Spirit then you will not be forgiven so long as you continue in this misattribution.” This means they remain in a state of having blasphemed. It does not mean they are continuing to blaspheme (ongoing action) because that would be present tense. Neither does it mean they have done it once for all completed and now it remains something they have done–this would be perfect tense. Dana & Mantey call this the Aorist of Occurrence. They have done it in the past and now they are continuing in this belief. So long as a person is in this status (of attributing divine acts to Satan) he will not be able to be forgiven. I believe that had these same scribes changed their mind and said, “I now believe this is done through the power of the Holy Spirit and that Jesus of Nazareth is Messiah, they would have been forgiven for everything, including for their earlier statement.” Of course, I cannot prove it, but this seems to work better with the rest of scripture. Consider Paul, for example.
However, this does not exhaust the understanding of this passage. We have to look at something else. Remember the context and what was happening. They were seeing direct miracles that should have been obvious to them as acts of the Holy Spirit. They were seeing these at the hands of Jesus. This was the setting. It does not appear that such an act is still possible today. We will not have Jesus standing before us doing obvious acts of God, so we will never be able to commit this sin. This should not to be equated with what we do today when judging the legitimacy of one claiming to do ‘signs and wonders.’ Scripture commands us to do this. Even the scribes believed they were doing this in obedience to scripture. The problem is the surety of what Jesus was doing in comparison to what any other miracle worker does today. He was yet to give them the greatest irrefutable proof—his own death and burial to be followed by his resurrection! They would see this evidence. Once they saw this, so long as they refused to accept his works as being those of God there would be no forgiveness for them. This actually works into the remnant scenario of Israel. Only those who accepted Christ remained branches in the tree of Israel (Romans 9-11). Those who still insisted his were the works of Satan would be cut off from the tree. Praise God! Romans goes on to say that any of them who change their view of Christ can easily be grafted back into the tree of Israel.
So the big thing to remember about the blasphemy of the Holy Spirit is that the context that defines the act will never be repeated, so you are in no danger of committing that act. For you today, there is no sin for which you cannot be forgiven. The only thing that gets in the way of forgiveness is rejection of the forgiver.