Jesus mentions this sine which is unforgivable in Matt. 12:31-32 and calls it blasphemy of the Holy Spirit. What exactly is this unforgivable sin?
Catechism of the Catholic Church (emphasis added):
1864 “Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.” 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.
Blasphemy against the Holy Spirit is to refuse God’s forgiveness. The prayer of absolution in confession states: “God the Father of mercies, through the death and resurrection of his Son, has reconciled the world to himself and sent the Holy Spirit among us for the forgiveness of sins…” God does not force himself on anyone. God does not forgive the sins of those who refuse the forgiveness offered through the Holy Spirit.