The unpardonable sin is simply defined as giving Satan the glory for work done by the Holy Spirit.

Matthew 12
³¹And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. ³²Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.

Hebrews 6
⁴It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, ⁵who have tasted the goodness of the word of God and the powers of the coming age, ⁶if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.

2 Peter 2
²⁰If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning.

Hebrews 10
²⁶If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, ²⁷but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. ²⁸Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. ²⁹How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace?

2 Peter 3
⁹The Lord is not slow in keeping his promise, as some understand slowness. He is patient with you, not wanting anyone to perish, but everyone to come to repentance.