Korihor was able to convince many to commit great whoredoms with the doctrine that there is no afterlife. Thus, the denial of immortality inevitably encourages immorality. This doctrine is a corollary to the doctrines that man will not be punished for his sins and that there is no hell. It is the "eat, drink and be merry" doctrine.
Joseph F. Smith
"Some people cannot think of anything else but annihilation. What a glorious prospect for the sinner! Then he could say, 'Let us eat, drink, and be merry, for to-morrow we die, and next day we will be annihilated, and that will be the end of our sorrow and of God's judgment upon us.' Do not flatter yourselves that you are going to get out of it so easy. This Book of Mormon is replete all the way through with the testimonies of the servants of God that men are born to be immortal; that after the resurrection their bodies are to live as long as their spirits, and their spirits cannot die. They are immortal beings, and they are destined, if they commit the unpardonable sin, to be banished from the presence of God and endure the punishment of the devil and his angels throughout all eternity. I think the wicked would prefer annihilation to the suffering of such punishment. That would be an end to punishment--an end to being. This view cannot be reconciled with the word of God." (Collected Discourses 1886-1898, ed. by Brian Stuy, vol. 4, Joseph F. Smith, Jan. 20, 1995)
Hugh Nibley
"That is no hope for anything future-no hope for any hereafter, that this is all there is. That's what nihilism is. There is no more; this is all there is. Don't expect anything else. As we said before, 'Una perpetua nox dermienda,' (One perpetual black night awaits us), and that's all. Since this is all there is, we act that way. That is nihilism." (Teachings of the Book of Mormon, lecture 8, p. 110)