Ammon is successful because of his great faith

Ammon is successful in teaching Lamoni because of his great faith in
Jesus Christ. Because of his faith, he had the strength to protect
the king's flocks and win the respect of the king. Because of his
faith, he was able to discern the thoughts of Lamoni say the right
words to teach him the gospel. Because of his faith, he was inspired
of the Lord. Alma 18:22 says, "Now Ammon being wise, yet harmless,
he said unto Lamoni: Wilt thou hearken unto my words, if I tell thee
by what power I do these things? And this is the thing that I desire
of thee."

See http://scriptures.lds.org/en/alma/18