Blasphemy Against The Holy Ghost
American Tract Society Bible Dictionary [1]
Matthew 12:31,32 Mark 3:28 Luke 12:10 This sin was committed by the Pharisees when they, in violation of their own convictions, willfully and maliciously ascribed the miracles of the Son of God and the work of the Holy Spirit to the evil one. It is often inquired whether this was the "sin unto death" spoken of 1 John 5:16 , and whether it is committed in these days. However these questions may be answered, certain it is that when one can ridicule religion and its ordinances, when he can make sport with the work of the Holy Ghost in the human heart, when he can persist in a willful disbelief of the Gospel, and cast contempt upon Christianity and "the ministration of the Spirit," he is going to a fearful extremity of guilt, and provoking the final withdrawment of divine grace. While on the other hand the vilest blasphemer, who feels the relenting of godly sorrow for his sins, and the desire to confess them at the Savior's feet, may be sure of realizing the truth of Christ's word. "Him that cometh unto me I will in no wise cast out."
Charles Buck Theological Dictionary [2]
See Unpardonable Sin