After setting up the conversion of the Lamanites as the example of a difficult and perhaps unimaginable conversion (at least to the Nephites), Samuel turns that example upon the currently apostate Nephites. They must repent, or else they will be destroyed. The Nephites have had mighty works done among them, and these great opportunities bring greater responsibility, and greater danger of destruction from the rejection of the truth.
Luke 12:48
48 …For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.
The Nephites fall into this category. The Lamanites do not. The Lamanites will be preserved, and the Nephites destroyed. For the Lord the difference is the opportunity they have had. The rejection of the Atoning Messiah by the Nephites is a much greater sin than the Lamanites’ prior disbelief. For the Nephites, they had to turn their back on the Savior. The Lamanites did not really know him.
Remember the story of Ammon before Lamoni. When Ammon begins to teach Lamoni he asks Lamoni if he believes in God:
Alma 22:7
7And Aaron answered him and said unto him: Believest thou that there is a God? And the king said: I know that the Amalekites say that there is a God, and I have granted unto them that they should build sanctuaries, that they may assemble themselves together to worship him. And if now thou sayest there is a God, behold I will believe.
Lamoni does not know that there is a God. He only knows that he has heard of God. Lamoni never rejected his God, he never truly knew Him. Therein is the difference. The Nephite knew, and rejected. It was this active turning away from the gift of understanding that brought the devastating penalty upon them.
Textual: There is no chapter break at this point in the 1830 edition.