Final unbelief is not essentially or principially different from daily unbelief. A person who has no heart for God is condemned while he lives. Dying impenitent does not worsen the sin, taking it to some new level where it becomes unforgivable. Dying is not a sin, it is a consequence of sin. Dying is not an aggravation of sin whereby the sin of impenitence becomes more heinous.
It is simply that the day of grace is past and that person’s impenitence will not be forgiven. But impenitence and unbelief are quite forgivable. In fact, God forgives everyone who is born again of both.
Just because a sin is not forgiven, does not mean it is unforgivable in principle. The sin against the Holy Spirit is a sin that cannot be forgiven in principle. Why? We do not know for sure, but that it is such a sin is clearly taught by our Lord.
The notion that all who die in unbelief have resisted the Spirit and therefore, blasphemed Him, requires one to assume the semi-Pelagian (Roman Catholic and Arminian) position that the Spirit strives to convert everyone; that the Spirit does all He can, short of violating a person’s free-will, to get everyone to convert. This is patently untrue. Therefore, any argument based on such a view is also patently untrue. There are many who die impenitent with whom the Spirit did not strive at all. He simply left them in their ignorance and hatefulness towards God.
It is true that sometimes the Spirit is said to strive with those who remain impenitent, but it is never taught in Scripture that this is always so. This non-saving work of the Spirit in the hearts of unbelievers is, I believe, just another way of saying that the Spirit accompanies the Word preached with convictions of conscience. Conscience still works because God’s common grace keeps it functioning to some degree or another. But, the Spirit working on the conscience is not the same thing as the Spirit doing all in His blessed power to convert the non-elect unbeliever. The Spirit could sovereignly give him the new birth, if it was God’s will to do so. Such strivings of the Spirit only serve, in the end, to make men more guilty of persisting in unbelief. Now resisting one’s conscience does aggravate a sin and make it more heinous, but that’s another issue for another day.
Thomas Chalmers sermons on blasphemy against the Holy Spirit here (pg. 332 and following) and here (pg. 43 and following) are both very helpful resources in understanding the passages that speak about the unforgivable sin.