There is much that is difficult to understand about the unforgiveable sin. In one place (perhaps only one application), Jesus is instructing his disciples about their future ministry. He says, “And everyone who speaks a word against [blasphemes] the Son of Man, it will be forgiven him; but he who blasphemes against the Holy Spirit, it will not be forgiven him” (Lk 12:10).
Since Jesus came to live as a perfectly responsive human (though He was God, Phil 2:11), the Spirit was working through Him in those powerful ways we find in the gospel accounts. In the next two verses Jesus reminds the disciples that this Spirit is the one who will provide the words they will say when they will find themselves under duress, and urges dependence on Him. This is the way the Son of Man had lived before, constantly trusting the Spirit.
I suspect that Jesus is saying therefore that the sin that is unforgivable has not happened when a person speaks against the humanity of Christ (as, for instance, Paul did prior to conversion), but when he has a determined and set resistance to His words, to the point of verbal rejection or blasphemy (indicating the high level of that resistance), since these are words and works that the Spirit gives Him. At what level indicates permanent rejection, only He knows. These words “will judge him on the last day,” He says in another place.
At least this is one reasonable way, in my view, to frame the idea of a sin that cannot be forgiven.