Rhetorical: Alma now begins to define how it is that the ordination of the priests after the order of the Son would create a type of the mission of the Son. To do this, he places them at the foundation of the world. While he certainly doesn't say they were in the Garden, placing them at the "foundation of the world" certainly locates them somewhere in the frame of that story. Thus he begins is justification of the priests that were ordained in precise parallel to the story he has already told of the Garden of Eden.
Next, he makes sure that these priests have agency. This is the same agency that Adam and Even had. Next, the blessing of becoming a priest is not universal, but restricted to those who have chosen good (and exercised great faith). To these is given a holy calling, or the priesthood.
Even this priesthood has a history. It too was "prepared." This is a frustrating paragraph in English, but is simplified by pulling it apart. The priesthood was prepared "with" the plan of redemption. The priesthood was prepared "according to" the plan of redemption.
What Alma is telling his audience is that the priesthood is intimately connected with the plan of redemption. It is gained upon principles of righteousness when good has been chosen. It is effective in the context of a plan that allows for repentance and redemption. He is telling them that this very priestly authority is designed as part of the plan of the Atoning Messiah, in whom they do not believe. Alma is undercutting their ability to have faith in their own lawyers, as this priesthood from ancient times is intimately bound with something that the lawyers deny.