The sons of Mosiah had the ability to speak with power and authority of God (17:3). Why is special attention drawn to this point? Evidently, these missionaries had been called, they had been set apart, and received power not only by fasting and prayer, but they also had been given the appropriate authority.
How does this apply to us when we receive a church calling? We too need to have the authority to act. How do we get that authority? We receive the authority of God by the laying on of hands. We should not serve in a calling without being set apart for that particular calling. Undoubtedly, every bishop would be happy if members who were recently sustained to new callings in sacrament meeting would come and ask for that setting apart and blessing. It is hard for bishoprics to keep track of this sort of thing. There is no tracking system on your membership record to indicate that you have or have not been set apart for each calling you receive. The bishop may assume that one of his counselors has set you apart for your calling, and the counselors may assume that the bishop has done it. We should be sure that we are set apart for any calling, and thereby receive the appropriate authority and blessing. We should also listen carefully to the contents of the blessing, and remember and strive to live worthy of its promises.
These four missionaries had the right to serve as God’s missionaries. It is not something that a person simply takes upon himself or herself. We may assume this happened by some laying on of hands (as was typically done anciently in consecrating a priest or making a sacrifice; see particularly, Moroni 2:2). Then, by their righteous living they were able to speak with authority. An important principle about the Melchizedek priesthood is that it has the power to cause people to repent (see Alma13:18). Righteous use of authority is the key for opening up the hearts of people so that they will repent. There are other dimensions to the priesthood, of course; but every purpose—everything we do in the priesthood, when you think about it—is to bring people to repent. We have no need to wonder why the Book of Mormon instructs readers so thoroughly about the preparations of the sons of Mosiah as they undertook their calling as missionaries. Through the power and the authority that these missionaries had, they were, most notably, able to bring about repentance in the lives of many people—Lamoni, Lamoni’s father, the Anti-Nephi-Lehies, and many individuals within these lands and kingdoms.