Being wise yet harmless (v. 22) is the same principle that Jesus taught his twelve apostles during his earthly ministry when he sent them on a mission. “Behold, I send you forth as sheep in the midst of wolves; be ye therefore wise servants, and harmless as doves” (JST, Matthew 10:14 italics indicates a change in the JST text). Ammon was a wise servant in the midst of “a wild and a hardened and a ferocious people” (Alma 17:14). He was wise in getting the king to commit to hearken to his words. He was harmless because the king would not be hurt, but only helped by following what Ammon would teach him. The king being “caught with guile” (v. 23) means he became obligated to listen through the strategy of Ammon. Strategy is one of several synonyms for guile.
Ammon illustrated the requirements for being an effective missionary as he spoke with the king. To believe in God, or in the Great Spirit, is a prerequisite for the spirit of prophecy. The spirit of prophecy is the testimony of Jesus (see Revelation 19:10). Ammon explained that the Great Spirit that the king believed in was God (Alma 18:26–28). Furthermore, Ammon bore testimony that God created the heavens and the earth, and of the heavens being the place where God and the holy angels dwell (vv. 28–32). He also bore testimony that God knows all the thoughts and intents of the heart (v. 32). King Lamoni received all that Ammon taught him by the spirit of prophecy. We can assume that the Spirit bore testimony of Ammon’s teachings and thus the king experienced the spirit of revelation, which is also a gift from God.
The king’s next question was about Ammon’s authority. Ammon bore testimony that he was called by revelation to teach the things he had been taught, which he had learned by revelation, and from his father. Ammon is an example of what Alma testified was recorded in his record (see Alma 17:3 and the superscription).