The covenants of the Father unto the house of Israel have been made to create the way to the eternal joy which is the fruit of the tree of life. The vision has been laying out the way that these covenants would work through the vicissitudes of the agency of humankind to reach the ultimate goal of bringing the house of Israel to its final goal.
At this point in the vision, the covenant peoples from each hemisphere have been united. The Gentiles have received the promise of prospering upon righteousness, and the joining of the New World book with the Old World book removes the impediment of lost understanding.
Now Nephi is shown that Jehovah will “work a great and marvelous work.” That work will be to restore the clear vision of the narrow path. Symbolically, the mists of darkness will be removed. The actual metaphor is to deliver the people from the “hardness of their hearts and the blindness of their minds.”
What had begun as a story of two hemispheres and two branches of the house of Israel now moves to the point where the story becomes the story of two choices: on the one hand life eternal, and on the other temporal and spiritual destruction.