Note 1 at Mt 12:31: Outside of total rejection of salvation, the blasphemy against the Holy Ghost is the only unpardonable sin spoken of in the New Testament. The fact that Jesus irrevocably damns those who blaspheme the Holy Ghost further attests to the divinity of the Holy Ghost and the teaching of the trinity (see note 2 at Mr 1:10).

The word "blaspheme" means "1. To speak of (God or a sacred entity) in an irreverent, impious manner. 2. To revile; execrate" (American Heritage Dictionary). In context, Jesus was saying that blasphemy against the Holy Ghost is attributing the working of the Holy Ghost to the devil (Mr 3:30). Many people in the Bible did this, including Saul, who became the Apostle Paul. However, we see in 1Ti 1:13 that Paul said he received mercy concerning his blasphemy because he had done it "ignorantly in unbelief." Therefore, the blasphemy against the Holy Ghost that Jesus was warning against must be willfully reviling the Holy Ghost with knowledge of what is being done.

This parallels Heb 6:4-6 where qualifications are placed on who can fall away from grace. That passage shows that only a mature Christian can commit such a thing. Likewise, with blasphemy against the Holy Ghost, rash statements spoken against the Holy Ghost in ignorance or unbelief by those who don't really know what they are doing can be forgiven.

From our human perspective, no clear line can be drawn as to when people become accountable for blasphemies and have committed this unpardonable sin. We can be assured that God knows the hearts of all people and that He will have righteous judgment concerning this. However, God's Word does show us that when people become "reprobate," they lose all conviction from God (Ro 1:28). Therefore, those who are convicted and repentant over having possibly blasphemed the Holy Ghost have not yet reached the place where it is unpardonable, or they wouldn't care.