"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men." (Matt 12: 31)
"They are they who are the sons of perdition, of whom I say that it had been better for them never to have been born." (D&C 76: 32)
LDS theology is, in some respects, nearly universalist in its ideas about salvation. While we certainly believe in divine punishment, we also believe in a healing afterwards for the overwhelming majority of humanity. Two of our midrashim come to mind here:
1) Elder Talmage used to say of Matt 5: 25-26, that the judge is God, and that as the uttermost farthing can be paid, so will the souls in prison be freed in course of time.
2) D&C 19 itself operates like a midrash. Among other things, it explains that "it is not written that there shall be no end to this torment, but it is written endless torment...For, behold, I am endless, and the punishment which is given from my hand is endless punishment, for Endless is my name." By shifting the meaning of endless in this context from chronological to possessive, the section affirms the Latter-day Saint belief that hell is a condition with an end as well as a beginning.
Even most murderers and heinous criminals, we believe, will eventually allow themselves to be healed enough, through the merits of Christ, to inherit some degree of glory. Everyone goes to one form of heaven or other.
Perhaps because agency (free will) is absolute in our belief system, there must be room left for those who would choose darkness in the face of overwhelming light. Thus, in Mormonism, it is possible to earn darkness without a chronological end; it is possible to commit a sin that cannot be forgiven. That's how we understand Matt. 12: 31, and why we have a fourth, non-kingdom described in D&C 76 along with the degrees of glory. We believe that some people, probably very few, will "sin against the Holy Ghost" and become "sons of perdition."
How do you know if you are one?
I probably never would have come up with this question on my own, because I was always satisfied with the folk Mormon answer that you have to have an awful lot of light and knowledge before you can sin against it that severely, and most of us simply don't.
On my mission, however, two of my favorite people were plagued with doubts that they had committed this sin, and were beyond all forgiveness.
The first was seventeen years old at the time, a son of atheists who had come to believe in nothing so firmly as his own spiritual experiences. It was because he experienced God so strongly and unexpectedly, I think, that he doubted himself so much when he acted in direct opposition to what he had felt. Perhaps in some moments of fear or doubt he denied to others that he had ever had spiritual experiences. In either case, he believed himself guilty of sinning against the Holy Ghost.
The second was perhaps twenty-three. She had joined the church at sixteen and was a sort of scriptural savant, memorizing whole passages with ease. She, too, felt the spirit with great intensity--then broke a series of commandments, stopped coming to church, and got involved in things that made her feel a strong sense of darkness. Did her turn from light to darkness constitute sinning against the Holy Ghost? Could she be forgiven for a turn she had made with such acute awareness?
Talking with these two made me think about the subject more deeply than I might have on my own. I was quite convinced, from the beginning, that neither was beyond forgiveness...far from it. And yet how could I give that confidence to them in a way that they could hold onto in moments of fear and self-doubt?
You can tell you are still eligible for forgiveness, I told the first, whenever you still have a desire to repent. There is no sin which God cannot forgive on the condition of repentance, but it is possible to shut out the Holy Ghost so completely that we are "past feeling" and thus lose all desire to repent. Sons of perdition could be forgiven, but will not be because having shut themselves completely off from the influence of the Spirit, they will never repent.