He already believed in God. He just didn't know it was God. The gospel did not give Lamoni a new faith — it gave a name to the one he already had.
The King Who Killed His Servants
The first thing the record lets us know about Lamoni is ugly, and we should not soften it. When the flocks of the king were scattered at the waters of Sebus, the servants who lost them expected to die, for the king had slain them because their flocks were scattered (Alma 17:28). This was not rage in the moment; it was policy. Lamoni had a settled habit of executing the men who failed him. The household Ammon walked into ran on fear.
So when Ammon recovered the flocks and the servants carried the severed arms of the raiders into the throne room, Lamoni's first reaction was not gratitude — it was dread. Surely this is more than a man (Alma 18:2). A man who rules by killing the people who disappoint him knows exactly what he deserves, and the appearance of someone with real power frightens him before it impresses him. He was afraid he had done wrong in slaying his servants (Alma 18:5). The fear of judgment was already in him. It just had no shape yet.
"Art Thou the Great Spirit?"
Here is what makes Lamoni worth a portrait. He was not an atheist, and he was not a man of no faith. He had a theology. His fathers had handed it down: there was a Great Spirit — a being above the world, who had power, who rewarded and punished, whom you did not want to cross. When the impossible thing happened at Sebus, Lamoni reached for the biggest category he had, and asked Ammon outright: Art thou the Great Spirit? (Alma 18:18).
It is one of the most quietly important questions in the Book of Mormon, because it shows a half-light. Lamoni believed in something true. He believed the universe was governed, that power belonged to a being greater than kings, that wrong would be answered. What he lacked was the name, the face, the relationship. His faith was real and almost entirely in the dark.
And watch what Ammon does with it. He does not tell Lamoni that his fathers' religion was nonsense. He does not demolish the Great Spirit and start over. He asks: Believest thou that there is a God? And Lamoni says, I do not know what that meaneth. And Ammon answers, Believest thou that there is a Great Spirit? — Yea. And Ammon says: This is God (cf. Alma 18:24–28). He takes the true thing Lamoni already held and tells him its real name. The Great Spirit you have feared your whole life — that is the Lord. Your faith was never wrong. It was only unfinished.
There is a moment exactly like this in the New Testament. Paul stands on Mars' hill in Athens, finds an altar inscribed TO THE UNKNOWN GOD, and tells the philosophers: Whom therefore ye ignorantly worship, him declare I unto you (Acts 17:23). That is the whole method with Lamoni. The Great Spirit is the unknown God of the Lamanites, worshipped in the dark for generations, and Ammon's gospel is not a foreign import but the lifting of a veil off a face Lamoni had been turned toward all along. The missionary's gift was not a new God. It was an introduction.
Two Days as One Dead
When the light came on, it overwhelmed him. Ammon taught him the creation, the fall, the plan of redemption, the coming of Christ (Alma 18:36–39), and Lamoni — a grown king, a hardened man — fell unto the earth, as if he were dead (Alma 18:42). They carried him to his bed and there he lay for two days and two nights (Alma 18:43). His servants prepared to bury him. The queen would not let them. She sent for Ammon, who knew what she did not: he is not dead, but sleepeth in God (Alma 19:8).
That phrase is the whole doctrine of rebirth in five words. The death-sleep of Lamoni is the same death-sleep Alma the Younger lay in for three days, the same that the great king his father would soon fall into, the same death-to-the-old-self that Paul names when he says we are buried with him by baptism into death (Romans 6:4). The man who ruled by death has to pass through a kind of death himself before he can be raised into the life that does not deal in it. Lamoni went down a king who killed his servants. The man who would rise had to leave that man in the grave.
"I Have Seen My Redeemer"
On the third occasion, the dark lifted. Lamoni rose, and the first words out of the reborn king were not about his kingdom or his power but about a person he had met in the dark:
Blessed be the name of God, and blessed art thou. For as sure as thou livest, behold, I have seen my Redeemer; and he shall come forth, and be born of a woman, and he shall redeem all mankind who believe on his name. (Alma 19:12–13)
I have seen my Redeemer. The Great Spirit has a face now, and the face is the coming Christ. The king who had been so frightened of judgment has met the One who answers it, and his fear has turned entirely to blessing. And then, full of it, he sank down again — and the queen sank down, and Ammon sank down, and the whole household was overcome by the Spirit at once (Alma 19:13–14), a court full of people lying on the floor of a palace, undone by joy, while outside a converted servant woman named Abish ran from house to house gathering the kingdom to come and see.
The Measure of the Change
You can take the measure of any conversion by what it costs and what it changes, and Lamoni gives us both. The man who casually executed servants becomes a man who, a few chapters later, loves Ammon as his own life and rides out at his side to rescue Ammon's imprisoned brothers (Alma 20). When his own father — the king over all the land — draws a sword to kill him for keeping company with a Nephite, Lamoni stands his ground. The convert who once ruled by fear now refuses to be ruled by it.
And the harvest did not die with him. His people became part of the Anti-Nephi-Lehies, the Lamanites who buried their weapons in the earth and would be slaughtered before they would take them up again — and of whom Mormon writes that as many as were brought to the knowledge of the truth … never did fall away (Alma 23:6). The king who used to scatter death over a scattered flock became the head of a people who chose to die rather than deal it. That is the distance the gospel moved him.
He believed in a Great Spirit he could not name and feared a judgment he could not escape. A servant in his stable turned out to be a missionary, and the missionary did not argue him out of his fathers' faith — he finished it, and gave the unknown God of the Lamanites the name Redeemer. Lamoni fell down as one dead and rose saying he had seen Him. The king who ruled by killing led a people who would rather be killed than kill. The whole arc is in that turn.