Ammon takes the first point of commonality, and then expands upon King Lamoni's response. Since Lamoni has opened the subject of heaven, Ammon continues to create a correspondence with what Lamoni would know. The place from which Lamoni's Great Spirit looks down upon mankind is equated to "the heavens."
King Lamoni process this information, and finds that it is easy to believe. After all, it really is little different (at this point) from what he already believes. He now pushes for the thing that is of interest to him, which is the source of Ammon's power. He therefore asks if Ammon comes from that God.