LUKE 12:10; MATTHEW 12:24-31; MARK 3:22-30
The word "blaspheme" means "to speak evil of; defame; or revile." Jesus is saying that blasphemy against the Holy Spirit is attributing the working of the Holy Spirit to the devil.
Many people in the Bible did this, including Saul, who became the Apostle Paul. In 1 Timothy 1:13, Paul said he received mercy concerning his blaspnemy because he had done it "ignorantly in unbelief." The blasphemy against the Holy Spirit that Jesus warns about must be the willful reviling of the Holy Spirit with knowledge of what is being done.
Hebrews 6:4-6, is where qualifications are placed on those who can fall away from grace. This passage indicates that only a mature Christian can commit such a thing. Blasphemy against the Holy Spirit; rash statements spoken against the Holy Spirit 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 someone becomes accountable for blasphemies and has committed this unpardonable sin. We can be sure that God knows the hearts of all men and that He will judge righteously concerning this. God's Word does show us that when anyone becomes a "reprobate," they lose all conviction from God and can care less what they think or say (Romans 1:28).
Anyone who is convicted and repentant over having possibly blasphemed the Holy Spirit has not yet reached the place where it is unpardonable or they wouldn't care.
Keep a tender heart and be sensitive to Him. Listen to His voice speak through His Word.