Apostasy is the highest form of blasphemy against the Holy Ghost because the Spirit has already bore witness in your spirit of the Truths about Jesus and you later deny them to be true.
Apostasy - Renunciation of Christ
The only unforgivable sin is the sin of apostasy - renouncing Christ.
It has been wrongly taught for years that not accepting Christ as your Savior is the “unforgiveable sin” and yes if you do not accept Christ as your Savior you will go to hell when you die, but it is not “technically” the quote – unquote “unforgivable sin” which is apostasy.
The renunciation of Christ, which is apostasy, is not something an atheist can do. Many people have a great misunderstanding about what apostasy is and what renouncing Christ is. The only type of person who can commit apostasy, who can truly renounce Christ, is a person who has known Christ. Therefore, a person would have to have been a Christian and then renounced Christ after becoming a Christian in order to commit apostasy.
Apostasy – I John 5 address what we must understand about Christ to be saved, and makes it known that God Himself bears witness of these Truths in our own spirit. Next the issue of praying for those who are in sin is brought into play and we told to pray for all sin except “sin leading to death” (NKJV) or more specifically “a sin unto death” (KJV) a reference to apostasy the highest form of blasphemy against the Holy Ghost because the Spirit has already bore witness in your (a Christian's) spirit of the Truths about Jesus and you (a Christian) later deny them to be true.
True biblical apostasy is a very rare thing to have happen.
Apostasy is a very difficult subject to fully understand and it takes a collection of the Scriptures to address the issue. That said, there is a portion of the book of Hebrews that covers this issue and then in I John 5we are told not to pray for this issue or a person so to speak who has committed apostasy. Note that Hebrews and I John are only a small portion of Scripture that touches on the issue of apostasy.
Please study1 John 5 and Hebrews 5, 6, and 7 with special attention to chapter 6. If you want to better understand it would be best to study the entire book of Hebrews and 1 John.
I would like to pour water on your fire. Renouncing Christ is not the unpardonable sin. You mention Hebrew 6. We need to look in detail at this sin.
Heb 6:4 -6 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
So we see from this scripture Paul makes a real effort to stress that the person he is referring to is a believer in Christ. "once enlightened", " tasted the heavenly gift".
Now we see the sin is "if they fall away", then they can not be restored. So initially like you say it sounds as if, if a christian leaves the faith they can not come back.
But let's explore the verse more fully. It says the person is "seeing they crucify to themselves the Son of God afresh, and put him to an open shame".
It says that this person who fell away is "crucifying Christ again". But think now how far does the cross span 1 sin, 2, 3. The bible says:
Heb 9:27-28 And as it is appointed unto men once to die, but after this the judgment: So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation.
It states as man "dies once", so Christ was "offered once". The cross spans our whole life time of failures.
So the person who has fallen away, was not saved at the point of death. The warning is still these for us, beware, don't think just because you are saved you always will be. You can fall away. But it is the point of death, not a denial of faith, that is unpardonable.
Many Christians in a time of pressure, may have denied Christ, as Peter did. It does not mean they can not be forgiven.
Jas 5:19-20 My brothers, if one of you has gone out of the way of the true faith and another has made him see his error, Be certain that he through whom a sinner has been turned from the error of his way, keeps a soul from death and is the cause of forgiveness for sins without number.