Alma retells the Liahona’s story, which must already have been familiar to Helaman. He was not giving his son new information but highlighting the aspects that he found most important. First, it was prepared divinely (“there cannot any man work after the manner of so curious a workmanship”). Second, the Liahona’s purpose was to show “the course which they should travel.” Third, it required faith to work correctly (“therefore they had this miracle”), that the miracles “were worked by small means,” and that a lack of faith stopped its working.
Alma is emphasizing Yahweh’s view of miracles. The Liahona’s workings were both essential and miraculous, but came from “small means.” This contrast is important—very small, perhaps insignificant, things led to great miracles. For Nephi, the “small thing” was the working of the spindles. For Alma, the “small thing” is the exercise of faith.