Blasphemy!

but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin. (Mark 3:29)

As a Catholic I have always been taught that this verse tells us that the only unforgivable sin is that of despair. To despair by turning one's back on God and refusing to accept that He can forgive our sins is in effect blaspheming His name. It is blasphemous to suggest that God is incapable of granting us forgiveness when we err. If we choose to turn our back on Him and deny Him, we will be left unforgiven and hardened of heart.