The standard answer for some one's concern about this is: "Well, if you're worried about it, you haven't committed it," which is probably the truth.
The scripture that concerns the unpardonable sin is found in Matthew 12:22-32. The actual verses that mention it are 31 and 32, but of course we should always read the entire story for proper context when we are looking for insight.
This is one of the many incidents of the Pharisees and their incessant search for some legitimate reason to condemn Jesus. This, of course, they could never find. But, in their jealousy and pride, they never stopped trying.
This was a case in which a man who could neither see nor talk (blind and dumb), due to a demon who had set up shop in him, was brought to Jesus. Christ healed the man, who then was able to see and talk. This absolutely enraged the Pharisees when they heard about it, and they accused Him of casting out demons through the power of Beelzebub (Satan). Jesus answered them in verses 25-31, and then spoke the words in v. 31-32:
"Wherefore, I say unto you, All manner of sin and blasphemy shall be forgiven men; but the blasphemy against the Holy Spirit shall not be forgiven men. And whosoever speaketh a word against the Son of man, it shall be forgiven him; but whosoever speaketh against the Holy Spirit, it shall not be forgiven him, neither in this age, neither in the age to come."
The 'blasphemy against the Holy Spirit' consisted in ascribing to Satan the work which had been done by the Holy Spirit. It was unpardonable because of the unusual set of circumstances out of which arose their rejection of Christ. It was the climax of their constant and continual denial that the miracles of Jesus represented the authentic power of God - which was the obvious truth, and seen as truth by thousands of other people - regular ordinary people.
The Pharisees, steeped in God's Word, and given the authority as religious leaders of the Jews, should have been the very first to recognize the Messiah. Instead, the unlearned had the insight to recognize Him, while the Pharisees in their pride and jealousy, committed deliberate, wanton apostasy by crediting to the devil the mighty works of Jesus by the Holy Spirit.
* unrepentant blasphemy
* by Jewish religious leaders - the shepherds of the Jews
* done purposely
* despite being personal witnesses to His miracles
* in the very presence of Jesus Christ, their Messiah!
These Pharisees had the highest duty and responsibility over all the people, and they would pay the ultimate price for their actions. There would be no pardon for such blasphemy.
I think it would be safe to say that they had the highest duty of any individuals - any religious leaders - who have lived in all of history. To them was given the sacred Word of God and the promise of their Messiah. When Christ was crucified as they insisted, they cried out, "Let it be on our heads." And it is.
To those to whom much is given, much is required.