Let's say a missionary goes to the jungle to bring the Good News to the savages, and explains John 3:16 to their elders. Through some translation error, they think it means Jesus has the power to grant literal immortality. They are delighted, and ask, "Can Jesus give us the power to bring the dead to life?" Let's say our missionary is well-read in the Bible but foolish, remembers that some of the disciples did indeed get the power to raise people from the dead, and tells that to the elders, perhaps even reads to the them from the book of Acts of the Apostles. The elders ask: "How can Jesus give us this power?" and the missionary stupidly says: "All you need is for me to baptize you." So they agree to be baptized, but quickly discover that they have not, thereby, acquired the power to raise the dead. Worse, they soon learn that the baptized are not immortal, and as luck would have it, the missionary has exposed them to European diseases for which they have no immunity, and they start dropping like flies. The elders then order the missionary to be put to death in some appropriate way, say, crucifixion. Have they commited the Unpardonable Sin of rejecting the Gospel after they have heard it? I don't think so. A better question: did the missionary commit the Sin against the Holy Spirit? Again, I think not. It was all a misunderstanding.
What about all the millions of people who watch American football on television, who have thus seen fans holding up the signs that say "JOHN 3:16"? Are they all damned to hell if they don't check out that verse and become "born again" Christians after reading the blessed promise: "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life."? I don't think so. The gifts of the Holy Spirit consist in more than just being flashed "Chapter and Verse" numbers. Inspiration means more than having that verse read, or even explained, preached, proclaimed and expounded.
As Paul told the Corinthians (1 Corinthians 12) the gifts of the Holy Spirit are diverse and vary from person to person. "To each is given the manifestation of the Spirit for the common good. To one is given through the Spirit the utterance of wisdom, and to another the utterance of knowledge according to the same Spirit, to another faith by the same Spirit, to another gifts of healing by the one Spirit, to another the workings of miracles, to another prophecy, to another the discernment of spirits, to another various kinds of tongues, to another the interpretation of tongues." The missionary in my story clearly got some of these gifts but not all of them. In truth, the Unpardonable Sin is rare, because the required gifts of the Spirit are rare, and rejecting them deliberately is even rarer still.
The Unpardonable Sin is something like the Story of Jonah, but not quite. Jonah gets the power of prophecy from God, but then is told to go use it in the City of Ninevah. Jonah refuses, and tries to run away from God. He ends up being tossed off a ship and swallowed by a whale. Eventually, the whale disgorges Jonah when he gets the message: you can run from God, but you can't hide. He goes to Ninevah and does what God told him to do. Now, evidently, running from God is not the Unpardonable Sin, because God has mercy on Jonah and gives him a chance to correct the error of his ways. Jonah would have committed the Unpardonable Sin if, after being given such a gift and being shown such mercy, he were to go to Ninevah and misuse the gift of prophecy by deliberately making false prophecies, out of some grudge or resentment for the way God had treated him.
The Unpardonable Sin is therefore something like what Nietzsche talked about, using the French word ressentiment. See On the Geneology of Morals, First Essay, Section 10. It is a poisoning of the Spirit by the retention of bad feelings about the coming to Self-consciousness through encounter with the Other. It is hatred toward God aroused by God's demonstration of one's inferiority to God, by giving you something which you would not otherwise have had.