What is the unpardonable sin?

The unpardonable sin is to blaspheme the Holy Spirit (Mark 3:28).  To blaspheme means to treat God disrespectfully through words or actions.  In the case with the Holy Spirit it is rejecting His call for you to accept Jesus as Lord and Savior.  The Holy Spirit testifies and proclaims our need for Jesus’ sacrifice and forgiveness (John 15:26).  If we reject His testimony of Christ then we are stating that we don’t need Him and that He is wrong, which is blasphemy.  So when someone rejects Jesus as Lord and Savior they are blaspheming the Holy Spirit, which is the unpardonable sin.