Unforgivable Sin is a haunting thing indeed. I don't really want to say too much about how is achieved by a human, rather than to point out what the book actually says about it. I've heard smart people say that "the one unforgivable sin is <arrogance or pride or doubting God... or whatever else they think it is>", and never refer to what the Bible says about it in the same sentence.
As if there is a commentary better than the gospels, which read:
Mathew 12:31-32
Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come .
Mark 3:28-30
Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter; but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation—because they said, “He has an unclean spirit” .
Luke 12:10
And anyone who speaks a word against the Son of Man, it will be forgiven him; but to him who blasphemes against the Holy Spirit, it will not be forgiven .
From this point: "blasphemy" can be deduced into a myriad of things; however, I believe that it's very important to quote these scriptures in ANY commentary that talks about how the Lord can be unforgiving.