Satan is called Perdition, meaning he is the author of ruination and the father of lies. Those who bask in the light of heaven and come to know God, and who then sin against that light and come to fight the faith of their fathers with a viciousness and a vengeance known only to the ungodly- these become the sons of perdition. Their sin is blasphemy, contempt for and defiance against the Holy Ghost and his witness. (Joseph Fielding McConkie and Robert L. Millet, The Holy Ghost, pp. 144-45.)
One who sins against the Holy Ghost- who has the heavens opened and comes to know God, and then denies that witness, turns sour to the sweet light of the gospel, and becomes an enemy and apostate to the cause of truth- that person is guilty of denying the Holy Ghost, the most serious sin in all eternity, an offense which shall not. be forgiven in this world nor in the world to come. This crime is called the unpardonable sin.
It is unpardonable in the sense that it is not covered or pardoned by the atonement of Christ, nor may any amount of the guilty person’s suffering here or hereafter atone for or pardon the pernicious deed and make up for the misery and suffering to the Saints which inevitably follow in its wake. (For further information on this most serious offense against God and man, see JST, Matthew 12:26-27; Hebrews 6:4-6; Hebrews 10:29; D&C 76:30-38; D&C 132:27; Teachings, pp. 24, 67, 156, 357-58; see also McConkie and Millet, The Holy Ghost, pp. 143-48.)