These are among the most celebrated and instructive scriptures on missionary work. The formula for teaching with the power and authority of God is explained succinctly: search the word of God, pray, fast, and teach by the Spirit.
Consider what is taking place: As Alma is journeying toward the land of Manti one day, he is astonished to encounter his beloved colleagues, the sons of King Mosiah, who are now on their way back to Zarahemla after having labored some fourteen years in a highly successful missionary campaign to reclaim the Lamanites. The Book of Mormon record at this point turns to a recounting of the extraordinary experiences of the sons of Mosiah. We learn the secret of their success: from the beginning they practice the godly regimen of feasting upon the word of God, praying diligently in the attitude of fasting, and thus cultivating within themselves the spirit of missionary work, in effect, the spirit of prophecy and revelation. The gateway for gaining this marvelous gift is personal dedication, scripture study, fasting, and prayer—which enable us to teach with the power and authority of God. These principles are illustrated in great detail in the actual missionary report of the sons of Mosiah, beginning with the experiences of Ammon, “being the chief among them” (Alma 17:18), who is to win the heart of King Lamoni and his court through the influence of the Spirit.
President Gordon B. Hinckley discusses the kind of devotion needed in missionary preparation:
[We must] cultivate in our homes a proper attitude toward missionary service. We enjoy the blessings we so greatly treasure because of those who have gone before us. Out of a sense of appreciation, out of simple gratitude, we should make an effort to extend these same blessings to others. Our young people have an obligation to prepare themselves for missionary service… .
One of the great compliments paid the Savior was that he taught as one having authority. The missionary who knows scripture and can quote it speaks with the voice of authority. It is not essential to memorize five hundred citations, nor even three hundred. Fifty well chosen verses of scripture will become a remarkably effective tool in the hands of a missionary. May I suggest that in our family night gatherings we make it a project to memorize one scripture citation a week pertinent to this work. At the conclusion of a year our children will have on their lips a fund of scripture which will remain with them throughout their lives… .
Live for the opportunity when you may go out as a servant of the Lord and an ambassador of eternal truth to the people of the world. “And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.” (Matt. 24:14.) This is our commission, and this is our obligation spoken anciently and reaffirmed in modern revelation. (Teachings of Gordon B. Hinckley [Salt Lake City: Deseret Book, 1997], 344–345)