The Bible says that blaspheming God or Jesus is forgivable, but to blaspheme the Holy Ghost is not forgivable.
Luke 12:10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
The difference between the Holy Spirit and the Holy Ghost is that the Holy Spirit is of God the Father (old covenant). The Holy Ghost is of Christ the Son of God (the new covenant).
But here is the deal on this where the blasphemy challenge to do this sin gets it wrong.
1) If a unsaved sinner can commit a sin that allows him to be eternally damned before he gets saved. Then that shows that sin itself is stronger than the shed blood of Christ. Which makes Satan more powerful. So that does not work.
2) The unsaved sinner is already headed for eternal damnation, so does blasphemy make him double damned? It sounds funny, but that is what it would be like if this were possible by the unsaved.
So you see it denies the power of Christ to forgive "all" sins so that the unsaved sinner can be saved. So the warning is actually to the saved.
Example: If a unsaved sinner goes to church, and goes forward to receive salvation. How would it make logical sense Biblically for the pastor to say: I know you have taken the blasphemy challenge. Because you told me that you did. Because of this salvation is denied unto you because your sin is stronger than the shed blood of Christ.
See how wrong that sounds? So the deception here is a deception within itself. Because if you believe the lie that you are locked into damnation because of what was done. Then the lie itself dams you because you will figure you can never be forgiven.
2thess 2:9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
11 And for this cause God shall send them strong delusion, that they should believe a lie:
12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
So basically, the Blasphemy challenge is a double deception. One where if you don't know the word and the power of Christ. Then some will believe the lie and by the actions of it. Be damned for being willingly ignorant. Because how many people will take their word for what they claim will happen to them through this sin, but never look to see the truth about it?
Basically, they are willingly damned because they choose to be willingly ignorant. So it's not the sin that is damming them. It's the belief in the power of that sin to deny salvation to those whom have not received it.
mk 3:28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:
sons of men = unsaved sinners.
mk 3:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
To be in "danger" of eternal damnation means the saved. Why? The unsaved are already damned so the danger of it is already a factor in their life. But the saved do not have that same factor of being damned, so this sin puts them in "danger" of being damned.
Example: Do you tell a person who has aids that they are in "danger" of getting aids?
So the word "danger" reveals to whom this warning goes out to.