Verse 12 is a transitional verse. It closed the rhetorical questions examined in the last comment for verses 9-12. It is repeated here because it is the important foundation for the next part of Nephi’s message. In verse 12, Nephi makes two points. The first is that Jehovah can do all things, and the second is that therefore they should be faithful.
Nephi’s arguments in verse 13 pertain to the future. He has decried Laman and Lemuel’s inability to learn faithfulness from their experiences to date. Nevertheless, he encourages faithfulness for the benefits to come. One of those is that they will have a land of promise. This is the first time that we know that Laman and Lemuel have heard of the promise. However, that is surely and simply the way that Nephi wrote the story. Lehi would not have kept that promise from his family. If Lehi were to tear away his family from the Israelites’ promised land, it would only be fair that Jehovah would lead him to another.
Laman and Lemuel desire to return to Jerusalem, but the whole reason that the family is in the wilderness is that Jerusalem will be destroyed. Thus, if they have faith in their father’s prophetic calling, they will be saved from that destruction. Nephi lays out their choice. Either they return to prophesied destruction, or they go forward to a land of promise.