ANSWER: Before we begin, it needs to be noted that ANY sin in which forgiveness is sincerely asked in the name of Jesus is not unpardonable and will be forgiven (Matthew 12:31 - 32, John 3:16, 1John 2:1 - 2, 3:5, etc.). Your question is closely related to the idea of what does it mean to blaspheme God.
It is interesting to note that the Old Testament broadly classified sin into two categories. The first category, and by far the largest, was those committed out of ignorance and weakness. The second are those committed 'with a high hand,' meaning those in direct defiance to God and his commandments. An example of this is the man found collecting wood on the Sabbath in clear defiance of the Eternal (Numbers 15:32 - 36). The two broad categories of disobedience in the New Testament are those that can be forgiven and those that cannot (the unpardonable sin).
The most controversial verses related to your question are in the gospels. Jesus taught about a special class of disobedience in his scathing rebuttal to some religious leaders. They accused him of performing miracles and casting out demons by the power of Satan RATHER THAN by the power of God. Christ warned them that blasphemy against God's spirit would NOT be forgiven.
|25. But Jesus, knowing their (the Pharisees') thoughts, said to them, "Every kingdom divided against itself is brought to desolation, and every city or house divided against itself shall not stand. 26. And if Satan casts out Satan, he is divided against himself. How then shall his kingdom stand? . . . every sin and blasphemy shall be forgiven to men except the blasphemy against the Holy Spirit; that shall not be forgiven to men . . . but whoever speaks against the Holy Spirit, it shall not be forgiven him, neither in this age nor in the coming age (Matthew 12:25 - 26, 31 - 32, HBFV throughout, see parallel account in Mark 3:22 - 29)|
We also find warnings related to this rebellious choice in the book of Hebrews.
4. For it is impossible for those who were once enlightened, and who personally obtained the heavenly gift, and became partakers of the Holy Spirit, 5. And who have tasted the good Word of God, and the powers of the world to come, 6. If they have fallen away, to renew them again unto repentance; seeing that they are crucifying the Son of God for themselves, and are publicly holding Him in contempt (Hebrews 6:4 - 8)
26. For if we willfully go on sinning after receiving the knowledge of the truth, there is no longer any sacrifice for sins, 27. But a terrifying expectation of inevitable judgment and of fierce fire, which will devour the adversaries of God . . . 29. How much worse punishment do you think he will deserve who has trampled underfoot the Son of God, and has regarded the blood of the covenant, with which he was sanctified, as an unholy thing, and has scorned the Spirit of grace? (10:26 - 27, 29)
What makes the sin Jesus discussed unforgiveable? It is a deliberate renunciation of God. It is a knowing and willful rejection, without sorrow or any regret, of Jesus' sacrifice and the gift of salvation. It is the ultimate choice to consistently disobey our Creator and forever refuse to repent. Satan the devil has NOT deceived those who have committed this kind of transgression. Those who commit this sin have 'seared' their conscience.
1. Now the Spirit tells us explicitly that in the latter times some shall apostatize from the faith, and shall follow deceiving spirits and doctrines of demons; 2. Speaking lies in hypocrisy, their consciences having been cauterized with a hot iron; (1Timothy 4:2)
It is not possible to renew a person who does this, even with the Holy Spirit. In short, forgiveness is not available because it is not nor will ever be asked for.
The unpardonable sin is not done by accident. It is not a single act per se, but rather the outcome of a process of knowingly rejecting righteousness for a period determined by God. It is not done through the same kind of temptation, deception and weakness that ALL humans are susceptible to (1Corinthians 10:13). In the final analysis, anyone who is worried that they might have committed such a sin likely did not. Those who do sin in such a fashion not only understand what they are doing but also, like the devil and his demons, do not worry about it and do not care one bit what anyone (including God) thinks about it!