Blasphemy against the Holy Spirit is essentially the willful rejection of God’s grace and forgiveness.
A Catholic Commentary on Holy Scripture explains, “Unlike other blasphemies or sins which might be partially excused by ignorance, passion or inadvertence, this was[/is] a sin of willful malice and blindness to the light. As long as such a mentality persists, pardon is impossible, not because of any limitation of God, but because those who are guilty of this sin refuse to respond to the promptings of grace” (p.912).
The Catechism of the Catholic Church (CCC) states, “There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss” (CCC 1864).
See also God’s Mercy.