Michael King writes that in the Old World, as the house of Israel was in the midst of destruction and dispersion, the Lord promised through his prophet Isaiah that in an "acceptable time," he would hear them, and "in a day of salvation," he would help them and preserve them, give them "for a covenant of the people," and cause them to "inherit the desolate heritages." The Lord promised them, "I will make all my mountains a way, and my highways shall be exalted" (Isaiah 49:8, 11). The "mountains" of the Lord would provide "a way" for all of Israel to one day receive the covenant.
In the New World, as a witness of the destruction of his own people, but being one who understood the covenant, Moroni boldly declared: "The eternal purposes of the Lord shall roll on, until all his promises shall be fulfilled. Search the prophecies of Isaiah" (Mormon 8:21-23). Moroni saw in the writings of Isaiah the Lord's plan to keep his covenant with a people that had been defeated and destroyed. Nephi, Jacob, and the Savior also used the writings of Isaiah to explain God's intent to fulfill his covenant with the house of Israel.
A study of Isaiah and the commentary provided by Book of Mormon prophets shows that four main threads are instrumental in the Lord's plan to fulfill his covenant. First, many of the house of Israel will die before receiving the covenant; second, Christ must come and open the way for all people to be taught the gospel and enter into the presence of God; third, "mountains" will provide a way whereby all, including the dead, may be brought forth to receive the covenant; and fourth, the Gentiles of the latter days will be instrumental in helping ancient Israel receive the covenant. [Michael L. King, "Isaiah's Vision of God's Plan to Fulfill His Covenant," in Covenants Prophecies and Hymns of the Old Testament, pp. 162-164]