Alma 13:6 - 13:10
Alma 13:6 And thus being called by this holy calling, and ordained unto the high priesthood of the holy order of God, to teach his commandments unto the children of men, that they also might enter into his rest—
Therefore they who have been given authority to preach the gospel by their priesthood ordination can now teach the rest of mankind so they too can return to God and enjoy eternal life.
Alma 13:7 This high priesthood being after the order of his Son, which order was from the foundation of the world; or in other words, being without beginning of days or end of years, being prepared from eternity to all eternity, according to his foreknowledge of all things—
So to accomplish this task of bring the gospel to the world, they were ordained to the order of the priesthood which has existed from even before the world was created. These ordinations were done because God knew who had a desire to do so do His work.
Alma 13:8 Now they were ordained after this manner—being called with a holy calling, and ordained with a holy ordinance, and taking upon them the high priesthood of the holy order, which calling, and ordinance, and high priesthood, is without beginning or end—
And their ordinations will last forever and they will never be released from priesthood responsibility. This is the way it has always been. There was no beginning and there will be no end of the priesthood and it’s roll in the salvation of our souls.
Alma 13:9 Thus they become high priests forever, after the order of the Son, the Only Begotten of the Father, who is without beginning of days or end of years, who is full of grace, equity, and truth. And thus it is. Amen.
And those who are ordained and remain faithful will be just like Christ in what He is trying to accomplish in relation to our eternal salvation. We will be filled with just as much grace and love for our brothers and sisters as He is.
Alma 13:10 Now, as I said concerning the holy order, or this high priesthood, there were many who were ordained and became high priests of God; and it was on account of their exceeding faith and repentance, and their righteousness before God, they choosing to repent and work righteousness rather than to perish;
Again to reiterate, many faithful brethren were called before the world was created to be ministers to us all. Why? Because they wanted to do that which would bring salvation to us all rather than doing the opposite which was to do that which could only bring disappointment and being cut off from all that was good and uplifting.