9 . . . they fasted much and prayed much that the Lord would grant unto them a portion of his Spirit to go with them, and abide with them, that they might be an instrument in the hands of God to bring, if it were possible, their brethren . . . to the knowledge of the truth . . .
11 And the Lord said unto them also: Go forth among the Lamanites, thy brethren, and establish my word; yet ye shall be patient in long-suffering and afflictions, that ye may show forth good examples unto them in me, and I will make an instrument of thee in my hands unto the salvation of many souls.
Ammon and the other sons of Mosiah pled with the Lord for help in their ministry to the Lamanites, and they became his "instrument" in teaching many the truth. What does it mean to be an "instrument in the hands of God?" One definition of the word instrument is "a mechanical tool or implement, esp. one used for delicate or precision work." Instruments - whether they are fine measurement tools, surgical implements, or musical instruments - are used by skilled individuals. They have to be calibrated, sharpened, and tuned. We prepare ourselves to be used by the Lord by calibrating our minds and hearts through the study of his word and by receiving instruction from him. We sharpen our focus through prayer and meditation. And we tune our hearts through righteous living.
The sons of Mosiah did all of these things. Mormon writes of their meeting with Alma after serving 14 years among the Lamanites:
2 Now these sons of Mosiah were with Alma at the time the angel first appeared unto him; therefore Alma did rejoice exceedingly to see his brethren; and what added more to his joy, they were still his brethren in the Lord; yea, and they had waxed strong in the knowledge of the truth; for they were men of a sound understanding and they had searched the scriptures diligently, that they might know the word of God.
3 But this is not all; they had given themselves to much prayer, and fasting; therefore they had the spirit of prophecy, and the spirit of revelation, and when they taught, they taught with power and authority of God.