In the counsel to his son Corianton, Alma indicates that the three most serious sins a person can commit on this earth are (1) the sin against the Holy Ghost, (2) murder—the shedding of innocent blood, and (3) unchastity. The Lord indicates the seriousness of all these sins in his revelations to Joseph Smith as recorded in the Doctrine and Covenants. The most serious of these sins is to deny the Holy Ghost and become a son of perdition. The Lord has defined this sin in these words:
Thus saith the Lord concerning all those who know my power, and have been made partakers thereof, and suffered themselves through the power of the devil to be overcome, and to deny the truth and defy my power—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;
For they are vessels of wrath, doomed to suffer the wrath of God, with the devil and his angels in eternity;
Concerning whom I have said there is no forgiveness in this world nor in the world to come—
Having denied the Holy Spirit after having received it, and having denied the Only Begotten Son of the Father, having crucified him unto themselves and put him to an open shame.
These are they who shall go away into the lake of fire and brimstone, with the devil and his angels—
And the only ones on whom the second death shall have any power. (D&C 76:31-37.)
This is an unpardonable sin because it cannot be paid for (or pardoned) either by the sinner himself or through the atonement of Jesus Christ.
The second most serious sin is to commit murder—that is, to willfully shed innocent blood. Concerning this sin, the Lord has said: “Thou shalt not kill; and he that kills shall not have forgiveness in this world, nor in the world to come.” (D&C 42:18.) Thus this is an unforgivable sin, which means that Jesus Christ cannot pay for (or “atone for” or “forgive”) the penalty demanded by the broken law. This sin is a pardonable sin, however; that is, the sinner can eventually make full payment himself, and be received into a state of pardon. Apparently one reason this sin is unforgivable is that forgiveness is based upon repentance, and a murderer cannot fully repent of his sin for he cannot make restitution of the life he has taken.
The third most serious sin is unchastity. The Lord has said, “Thou shalt not commit adultery; and he that committeth adultery, and repenteth not, shall be cast out. But he that has committed adultery and repents with all his heart, and forsaketh it, and doeth it no more, thou shalt forgive; But if he doeth it again, he shall not be forgiven, but shall be cast out.” (D&C 42:24-26.) Thus adultery is both pardonable and forgivable, but if committed again after a person understands the law it is unforgivable.
The word pardon as used in the scriptures means to be sanctified, to be clean, to reach a point where a broken law has no further claim upon the sinner. Thus the most serious sin is unpardonable because the law will always have a claim upon the sinner, and the sinner will always remain unclean; he cannot ever regain the presence of God, for “no unclean thing can enter into his kingdom.” (3 Nephi 27:19.) All unpardonable sins are of necessity also unforgivable.
The word forgiveness as used in the scriptures indicates one is “given something before.” Thus when a person repents of a sin, Jesus Christ, through his atonement, pays for (or atones for) part of the broken law before the person makes full payment. Thus he is “fore-given” that part of the penalty paid for by Jesus Christ. Forgiveness is possible only upon repentance; thus those who refuse to repent “remain as though there had been no redemption made, except it be the loosing of the bands of death.” (Alma 11:41; read also D&C 19:15-20.)