Alma summarizes his argument. Both the high priesthood and the mission of the Son himself were ordained from the foundation of the world. The need for redemption is part of the blessing of agency, and that is an eternal principle “being without beginning of days or end of years.” Although that phrase is unique to the Book of Mormon, it echoes language associated with Melchizedek, who had “neither beginning of days, nor end of life” (Hebrews 7:3).
Where the book of Hebrews in the New Testament associated the phrase with Melchizedek, Alma associates it with the priesthood. This is because there is a nonlineal, king-based priesthood associated with Melchizedek, and that is the type of priesthood available in the New World.
Thus, the priests who participate in the priesthood of the Son will share that priesthood forever. The priesthood is eternal, just as the Son is eternal.
The final “Amen” closed a chapter in the 1830 edition of the Book of Mormon. The next verse continues the discussion of the priesthood that is associated with Melchizedek, so it isn’t unusual for Orson Pratt to see the next verses as part of the same story. However, that wasn’t the way Mormon wrote. The testificatory Amen closes chapters, even if the story or sermon isn’t over. We therefore often see the end of the story written at the beginning of the next chapter, as we do here.