Alma 39:6. What is the unpardonable sin?

The Prophet Joseph Smith (1805–1844) gave further knowledge about the unpardonable sin:

“All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him. After a man has sinned against the Holy Ghost, there is no repentance for him. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it; and from that time he begins to be an enemy. This is the case with many apostates of the Church of Jesus Christ of Latter-day Saints” (Joseph Smith, in History of the Church, 6:314).

An individual who receives a witness of the Holy Ghost and then falls away or becomes less active in the Church is not guilty of the unpardonable sin.