Alma again uses the connection between the priesthood of Melchizedek and the Son of God as a proof of the Son/Messiah. His point is not to use Melchizedek to justify Jesus’s authority, as Paul does, because Jesus’s earthly mission had not yet occurred. For Alma, it isn’t the priesthood per se that is important, but rather the connection between the priesthood and righteousness.
Alma specifically associates the Melchizedek Priesthood with atonement: “Now these ordinances were given after this manner… that they might look forward to him for a remission of their sins.” This association reflects what appears to be the oldest understanding of the role of a Melchizedek priest. Margaret Barker has reconstructed the nature and function of this preexilic priesthood:
There were two rituals exclusive to the ancient high priests: entering the holy of holies with the blood on the Day of Atonement and consuming the bread of the Presence. Since these two are closely linked to the elements of the Eucharist, it seems likely that the high priestly traditions are the ultimate source of the imagery.… The Elephantine texts, which give a glimpse of Jewish life in Egypt in the sixth and fifth centuries B.C.E. often mention priests but never Aaron, nor Levi nor the Levites. Any rites and duties associated with Aaron probably came from the older royal priesthood of Melchizedek.
Specifically, she notes: “These three extracts, from Deuteronomy, the Assumption of Moses and the Melchizedek text, are mutually consistent, and show that the heavenly high priest was the Lord who came from his holy place on the Day of Atonement in order to save his people from the power of the fallen angels, to punish their enemies and kpr the land. I suggest, in the light of this, that kpr has to mean restore, recreate or heal.” The Melchizedek figure functioned in the temple in relation to the Day of Atonement, and the priest’s actions on that holy day effected the communal atonement. This atonement is, of course, a type of the Atoning Messiah’s mission. The connection that Alma makes between Melchizedek and the Messiah’s atonement is therefore entirely appropriate to the time period of the brass plates. (The Elephantine texts date to the fifth and sixth centuries before Christ.)