What exactly is the sin against the Holy Ghost? In the Church of Jesus Christ of Latter-day Saints a few of our prophets have described it as follows:
So what does it mean to blaspheme against, or deny, the Holy Ghost? Here’s what Joseph Smith said about it:
“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.”
President Spencer W. Kimball also added that,
“The sin against the Holy Ghost requires such knowledge that it is manifestly impossible for the rank and file to commit such a sin.”
Lucifer and his followers who rebelled against God in our premortal existence committed the unpardonable sin. They, knowing the truthfulness of God came out in open rebellion against him knowing full well that God exists and that He was God. In this life it is possible for some to come to this same knowledge about God and receive the strongest witness to their souls from the Holy Ghost that God the Father Lives, that Jesus is the Christ and then rebel against God and turn totally against him. Cain is an example of this. He had a full knowledge of God and his plan of salvation for us. Yet, Cain sided with Satan/Lucifer and turned against God. God even named Cain Perdition the same as Lucifer. All those who reach to such heights of knowing God and then rebelling against him commit the unpardonable sin and become sons of Perdition.
Doctrine an Covenants 76:25-27
25 And this we saw also, and bear record, that an angel of God who was in authority in the presence of God, who rebelled against the Only Begotten Son whom the Father loved and who was in the bosom of the Father, was thrust down from the presence of God and the Son,
26 And was called Perdition, for the heavens wept over him—he was Lucifer, a son of the morning.
27 And we beheld, and lo, he is fallen! is fallen, even a son of the morning!
Moses 5:22-26
22 And the Lord said unto Cain: Why art thou wroth? Why is thy countenance fallen?
23 If thou doest well, thou shalt be accepted. And if thou doest not well, sin lieth at the door, and Satan desireth to have thee; and except thou shalt hearken unto my commandments, I will deliver thee up, and it shall be unto thee according to his desire. And thou shalt rule over him;
24 For from this time forth thou shalt be the father of his lies; thou shalt be called Perdition; for thou wast also before the world.
25 And it shall be said in time to come—That these abominations were had from Cain; for he rejected the greater counsel which was had from God; and this is a cursing which I will put upon thee, except thou repent.
26 And Cain was wroth, and listened not any more to the voice of the Lord, neither to Abel, his brother, who walked in holiness before the Lord.