What is the unpardonable sin?

The Bible tells us that there is only one sin that cannot be forgiven, and that is to blaspheme the Holy Spirit (Mark 3:28-29).  How does a person do this?  The Holy Spirit points to Jesus Christ as the Lord and Savior of the world (John 15:26).  Since the Holy Spirit is the one that shows us how God has offered salvation to the world, we have to trust that He is telling us the truth.  By rejected Jesus as Lord and Savior a person is saying that the Holy Spirit is lying about God’s salvation and this is blasphemy.  Because of this the Holy Spirit has the power to reprove the world of sin, righteousness, and judgment and through these a person condemns themselves to Hell (John 16:7-11).