I imagine SZonian is thinking of this passage from the King Follett Sermon:
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.
Now,
is a mormon doctrine. I'm sure that we will see denials that Joseph Smith really meant that mormon "apostates" were sons of perdition, but those arguments are spectacularly unconvincing. Mormons I have known certainly accepted Smith at his word.
We first heard about the plan of salvation before we were born, in what the scriptures call our first estate (see Abraham 3:26). What occurred in this first estate is dimly understood, but we do know that we lived there as spirits, children of our Heavenly Father, and we made certain steps of advancement to prepare for the opportunity of housing our eternal spirits in earthly bodies. We also know that our Father held a great council to explain the purpose of earth life. We had the opportunity of accepting or rejecting the plan of salvation. It was not forced upon us. The essence of the plan was that man would have an opportunity of working out his own salvation on earth, with God’s help. A leader was selected to teach us how to follow the plan and to redeem us from sin and death. As the Lord explained to Moses, “Behold, my Beloved Son, which was my Beloved and Chosen from the beginning, said unto me—Father, thy will be done, and the glory be thine forever” (Moses 4:2).
Salvation by Grace through faith Placemarker
Let me clarify, since I can see where my statement could be misconstrued.
As Joseph Smith states in the quote you provide, “He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him”.
Leaving Mormonism does not mean you will be a Son of Perdition. Not everyone who leaves Mormonism fights against the church after having the heavens opened unto them.
My belief is that to be a Son of Perdition, you have to willingly and knowingly fight against God.