The consequences of committing the unpardonable sin are cataclysmic; it not only means that your resurrection will be delayed for an additional 1,000 years after the Second Coming of Jesus Christ, but that when you are resurrected (at what other Christians refer to as the "Resurrection of the Unjust"), you will be found to be "filthy still", and exiled to eternal darkness with Satan and his devils, with a resurrected body of no glory. The celestial, terrestrial, and telestial systems of glory will be eternally beyond your reach.
Let's see if we can better quantify the "unpardonable sin" and make the concept less scary. To further understand the nature of this sin, let's first present a description as posted on the LightPlanet website:
NOTE: References to TPJS are for "The Teachings of the Prophet Joseph Smith":
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).
This description does provide more insight, but is still primarily conceptual in nature. What does the phrase "crucify afresh" really mean? What does it mean to have the "heavens opened up to you"? We still are not presented with a list of specific actions necessary to make one a "son of perdition", nor can we yet nail down who specifically qualifies.
But all is not lost. Enter Duane Crowther, stage right. Crowther is an active Mormon who is a prolific author, writing on matters of doctrine and faith. One of his most influential books is entitled "Life Everlasting", which is described as a definitive study of life after death. There are two editions; the most recent was published in 1998 and is the edition I'll be referring to later in this post.
In this book, Crowther presents definitive answers to two questions; first, what is the unpardonable sin, and second, who is capable of committing it. His premise is that the unpardonable sin is not a single one-time act, but a series of actions which collectively can place one "beyond the pale". Furthermore, Crowther also suggests that only Latter-day Saints who possessed the Melchizedek Priesthood could ever truly commit the unpardonable sin and become "sons of perdition" because only they could ever be exposed to sufficiently qualifying enlightenment. This, of course, is only his own educated opinion based upon his interpretation of applicable doctrinal guidance, and does NOT necessarily represent the official opinion of the LDS Church leadership.
Unfortunately, the text of this book is not available online. Consequently, I am transcribing the information directly from the book. First, I will cross-post his guidance on the unpardonable sin itself, which begins on page 418 of the book. Some of the descriptions will be abridged:
Those who are candidates judged to have committed the unpardonable sin must do the following:
1. They deny the truth (D&C 76:31). They must reject the gospel and their testimony, though they have known of its truthfulness. Their denial must be an outright rejection of righteousness.
2. They deny the Holy Spirit after having received it (D&C 76:35).
3. They speak against the Holy Ghost (Matthew 12:31-32).
4. They withdraw themselves from the Spirit of the Lord (Mosiah 2:36). They reject that Holy Being, speak irreverently of the guidance He gives to man, and impiously deny that such a Being has ever manifested Himself unto him. They must reject Him as their guide and revelator.
5. They deny the Only Begotten Son of the Father (D&C 76:35,43).
6. They put Christ to an open shame (D&C 76:35)
7. They assent unto Christ's death (D&C 132:27)
8. They crucify Christ unto themselves (D&C 76:35). ...[They] come out in rebellion against Christ and desire to thwart His Church. In their rebellion they become so wretched that in effect they rejoice in the Savior's suffering and crucifixion and affirm that they would put Him to death if He lived in their day.
9. They defy Christ's power (D&C 76:31)
10. They come out in open rebellion against God (Mosiah 2:37).
11. They turn from and break the oath and covenant of the priesthood (D&C 84:41). ...[They] come out in full rebellion against the power of Christ as it is manifested on the earth. They oppose priesthood authority and actively fight against the Church. They seek the downfall of its leaders and attempt to bring misery to all who are following the paths of truth and righteousness. It is this spirit of opposition and rebellion that removes a disgruntled member of the Church into the category of apostasy and into the fate of the sons of perdition.
12. They suffer themselves through the power of the devil to be overcome (D&C 76:31).
13. They become an enemy to all righteousness (Mosiah 2:37). Those guilty of this sin "listeth to obey the evil spirit" and "remaineth and dieth an enemy to God" (Mos. 2:37,38). They have come to love Lucifer and hate Christ...
14. They commit murder in that they shed innocent blood (D&C 132:27). For murder to be classed as a portion of the unpardonable sin against the Holy Ghost, it must be committed by one who has been "sealed by the Holy Spirit of promise" after he has received the new and everlasting covenant. [Ed. Note: By "murder", we mean what is called "capital murder" or "first degree murder", deliberate and premeditated. Murder by negligence or spawned by passion may not necessarily qualify as murder in this context.]
In conclusion of this segment, Duane Crowther opines that the above 14 elements, drawn from the scriptures, together comprise the unpardonable sin, or blasphemy against the Holy Ghost. No single one of them alone is "unpardonable". But to commit one of them causes man to become entangled in Satan's web, and before long he may be led farther and farther down until he is guilty of all of them and is beyond the powers of redemption and forgiveness.
In the next post of this series, "LDS Doctrine: The "Unpardonable Sin", Part II - Who's Capable Of Committing The "Unpardonable Sin"? Duane Crowther's Perspective", I will discuss Duane Crowther's assessment of who is capable of committing it. Since this post was getting rather long, I decided to break the series into two posts.