Verse 20 quotes the words of Moses in Deuteronomy 18:15: "A prophet shall the Lord your God raise up unto you, like unto me." Who was the prophet that was like unto Moses that the Lord raised up? First, in the meridian of times, Jesus Christ fulfilled this prophecy. This is the most often quoted messianic prophecy in scripture, and has been quoted by Nephi, Peter, Stephen, Moroni and Jesus Christ himself. As Moses was a miracle worker, a redeemer, a deliverer, a liberator, a mediator of the covenant, a law-giver, a revelator, a prophet, priest and king—in each of these ways and in many others—the Lord Jesus Christ is even a greater prophet than was Moses.
In verse 21 Nephi declares that "this prophet of whom Moses spake was the Holy One of Israel." In verse 22 we read that the "righteous need not fear." Verse 24 says that "the righteous must be led up as calves of the stall, and the Holy One of Israel must reign in dominion, and might, and power, and great glory." That is really going to happen in the Millennium, when Jesus reigns as king of kings and lord of lords.
Verse 25 states that "he gathereth his children from the four quarters of the earth." In other words, all of Israel will be gathered, including any remnants of the Ten Tribes. Israel is scattered among all nations; Israel will be gathered from all nations. And verse 26 notes that it is "because of the righteousness of his people" that "Satan has no power." This will be during the millennium, and Satan "cannot be loosed for the space of many years." He is bound by the power of God so that the Millennium can begin. Then the people are righteous and Satan is bound by the righteousness of the people and by the power of the priesthood. Then, when the thousand years of the Millennium is ended, he is loosed for a little season.