From The Encyclopedia of Mormonism
Author: Turner, Rodney
The gravest of all sins is blasphemy against the Holy Ghost. One may speak even against Jesus Christ in ignorance and, upon repentance, be forgiven, but knowingly to sin against the Holy Ghost by denying its influence after having received it is unpardonable (Matt. 12:31-32; Jacob 7:19; Alma 39:6), and the consequences are inescapable. Such denial dooms the perpetrator to the hell of the second spiritual death (TPJS, p. 361). This extreme judgment comes because the person sins knowingly against the light, thereby severing himself from the redeeming grace of Christ. He is numbered with the sons of perdition (D&C 76:43).
The Prophet Joseph Smith explained, "No man can commit the unpardonable sin after the dissolution of the body, nor in this life, until he receives the Holy Ghost" (TPJS, p. 357). To commit the unpardonable sin, a person "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 deny Jesus Christ when the heavens have been opened to him, and to deny the Plan of Salvation with his eyes open to the truth of it" (TPJS, p. 358; cf. Heb. 10:26-29).
If people have such knowledge and willfully turn altogether away, it is a sin against light, a sin against the Holy Ghost, and figuratively "they crucify to themselves the Son of God afresh, and put him to an open shame" (Heb. 6:4-6; D&C 76:35). Such remain as though there were no Atonement, except that they shall be resurrected from the dead (Alma 11:41). RODNEY TURNER