The real sin of blasphemy against the Spirit is often misunderstood. The word "blasphemy" evokes thoughts of railing, cursing, profanities, or sacrilegious talk. At least, this is what the meaning of the word evolved to be. Let's look at the Biblical context.
When Jesus said:
Jesus saith unto him, Thou hast said: nevertheless I say unto you,
Hereafter shall ye see the Son of man sitting on the right hand of
power, and coming in the clouds of heaven. (Matthew 26:46, KJV)
The high priest's response was:
Then the high priest rent his clothes, saying, He hath spoken
blasphemy; what further need have we of witnesses? behold, now ye have
heard his blasphemy. (Matthew 26:47, KJV)
What could be even close to "blasphemy" by our modern standards? Yet the high priest claimed Jesus had spoken blasphemously.
When Jesus saw their faith, he said unto the sick of the palsy, Son,
thy sins be forgiven thee. (Mark 2:5, KJV)
The scribes' response:
But there was certain of the scribes sitting there, and reasoning in
their hearts, Why doth this man thus speak blasphemies? who can
forgive sins but God only? (Mark 2:6-7, KJV)
Jesus had offered the palsied man forgiveness for his sins. Only God can forgive sins, so what the scribes thought was correct. But Jesus was so obviously a human, and not God, standing there among them that they took his words to be blasphemy in that they were a claim to deity.
Jesus, however, was speaking the Father's words; they were the words of God. The scribes did not wish to acknowledge this, and railed against it. Ironically, they were committing blasphemy, by speaking against the Spirit. They did not want to accept the truth that Jesus was the Messiah. They persistently refused to accept this truth.
Fast forward to the next chapter in Mark.
28Verily I say unto you, All sins shall be forgiven unto
the sons of men, and blasphemies wherewith soever they shall
blaspheme: 29But he that shall blaspheme against the Holy
Ghost hath never forgiveness, but is in danger of eternal damnation.
(Mark 3:28-29, KJV)
Why are these sins never forgiven?
Consider the condition of the high priest and of those scribes who blasphemed against the Spirit. Were their sins ever forgiven? Why not?
It is because they had so persistently rejected the truth, that they were in a self-deceived state of mind in which they saw no need of forgiveness. They did not repent, confess their errors, and seek Jesus' forgiveness. That is why they could not be forgiven.
And this helps us to understand exactly what Jesus meant by the use of "blasphemy." Essentially, it boils down to one thing: Persistent rejection of truth; i.e. speaking against the truth.
The Spirit teaches truth. Rejecting the truths of the Spirit, and adding our voice against them, is to commit blasphemy against the Spirit. There is no unforgivable sin--but there are sins which will never be forgiven because no forgiveness is sought. If we do not repent, confess our sins, and ask for forgiveness, we remain in an unforgiven condition.
So when theologians conclude that blasphemy means a refusal to repent, they are on the right track. It might be more accurate to indicate that by speaking against the truth, one becomes self-deceived into actually believing that said truth is wrong--and it is this self-deception that prevents one from ever seeing a need for repentance.