The key phrase here is “sin willfully.” The author is describing an overall mindset rather than a single action. Many times when we sin, we have a willingness to sin because we give in out of weakness and do what we know is wrong. But willful sin occurs when a person expresses deliberate and sustained opposition to God and His law, and his heart has hardened enough that he defiantly refuses to repent. In this regard, the unpardonable sin is not a specific sin. Rather, it could be any sin that is committed with a heart that is against God and that refuses to soften.
The Bible shows a number of sins against the Holy Spirit that still fall short of willful blasphemy. Ephesians 4:20 speaks of “grieving” the Holy Spirit. Acts 7:51 mentions “resisting” the Holy Spirit. I Thessalonians 5:19 warns against “quenching” the Spirit. All of these show some opposition to the outworking, the power, and the fundamental nature of God.
But blaspheming the Holy Spirit ratchets up this opposition to the point that the things of God are deliberately despised and denigrated after receiving knowledge of the truth. It has the effect of trampling the very Creator underfoot and belittling the holy covenant of which He is the Mediator. Repentance is impossible because self-confidence (as opposed to faith in God) has hardened into an arrogant and insolent refusal to recognize God's preeminence. The rejection of God becomes so complete that the very idea of repentance becomes ludicrous. By rejecting the Spirit of grace and the forgiveness it allows, the blasphemer has nothing with which to pay for his sins, except his own life.
This condition can come about in a couple of ways. One is through deliberate choice. In this regard, among the biggest dangers to our walk with God is resentment and bitterness because these emotions can poison the mind to such a degree that we can simply stop caring about God and His way. The object or circumstance of resentment begins to take up more of our view—more of our thoughts—than God Himself, and our inclination toward His will becomes overthrown in the internal rage.
A second way is through spiritual neglect, the path these Hebrews were treading. Through neglect, God's truth slips away over time, and the things of Satan's world begin to fill the void. The result is such spiritual weakness that what truly matters is no longer a part of the reasoning process. God's law becomes unimportant, and Christ's sacrifice becomes irrelevant, like distant memories with no immediate value.