Although the Lord has not provided an extensive list of sins in the order of their severity, he has indicated the following as the three most grievous or serious sins a person can commit on this earth: (1) the unpardonable sin of denying the Holy Ghost once it has had place in you, (2) murder, or the shedding of innocent blood, and (3) unchastity. The unpardonable sin will be discussed in greater detail in the next note. Concerning murder, the authorities of the Church have published the following brief statement:
One of the most serious of all sins and crimes against the Lord's plan of salvation is the sin of murder or the destruction of human life. It seems clear that to be guilty of destroying life is the act of "rebellion" against the plan of the Almighty by denying an individual thus destroyed in mortality, the privilege of a full experience in this earth-school of opportunity. It is in the same category as the rebellion of Satan and his hosts and therefore it would not be surprising if the penalties to be imposed upon a murderer were to be of similar character as the penalties meted out to those spirits which were cast out of heaven with Satan. (Quoted in Doxey, The Doctrine and Covenants Speaks, 1:240.)
The teaching of the Church is equally strong on the sin of unchastity or adultery, as is indicated by the following statement by Joseph Fielding Smith:
I call your attention to this statement by the Prophet Joseph Smith: "If a man commit adultery, he cannot receive the celestial kingdom of God. Even if he is saved in any kingdom, it cannot be the celestial kingdom."Of course, a man may, according to the Doctrine and Covenants, 132:26, receive forgiveness, if he is willing to pay the penalty for such a crime: that is he "shall be destroyed in the flesh, and shall be delivered unto the buffetings of Satan unto the day of redemption," which is the time of the resurrection. We cannot destroy in the flesh, so what the Lord will require in lieu thereof, I do not know. . . .
We have been taught that adultery is a crime second only to the shedding of innocent blood. We cannot treat it lightly. For a man to destroy another man's home is too serious an offense to be readily forgiven. Such a man should not be permitted to come back in the Church, under any circumstances, at least until years have elapsed. He should be placed on probation for that length of time to see if he can, or will, remain clean. Even then I confess I do not know what disposition the Lord will make of him. To permit him to come back within a short time has a very evil effect upon other members of the Church who begin to think that this enormous crime is not so serious after all. . . .
If a man thinks the Lord has placed upon him a seal by which he is exempt from his transgressions and is bound to inherit eternal life, no matter what he does, as long as he does not commit murder, or shed innocent blood, or deny the Holy Ghost, he is in the gall of bitterness, as a Nephite prophet would have said.
No man is promised salvation who is not cleansed from all his sins, and if a man sins deliberately, thinking he is exempt, he will be damned! (Doctrines of Salvation, 2:93-94, 99.)