Old World Context: The basic instruction of this teaching is embodied in the paralleled statements in verses 19 and 20. In both cases there is a treasure “laid up.” In both cases, the relationship of that treasure to “moth” and “rust” is given. Of course the intent is to show the differences among the parallels rather than the parallel.
The treasure that is “laid up” on earth is necessarily an earthly treasure. It is used as the foundation of the metaphor because it would be so common for a poor people to dream of such treasures. Modern saints are no less susceptible to this temptation than our ancient forbearers. In some ways our plight is even worse because the advent of purchases on credit allow us to possess such treasures even when we cannot currently afford them. Thus our interest in these earthly treasures is not simply one of longing, but of current possession.
The fundamental contrast in the teaching is the nature of the things that are treasured. For earthly treasures the imagery suggest perishability. The treasures are accumulated goods, where fancy clothing might be destroyed by such natural causes as moths. This is contrasted to a heavenly treasure that is permanent.
There are, of course, differences between the earthly and heavenly treasures. The earthly treasures are valuable and desirable while on this earth, but they are also temporary. They cannot last. Heavenly treasures last forever. The problem, of course, is that they do not appear to be of as much usefulness during this life. As with many of the Beatitudes, Jesus teaches that we should look forward to the rewards of heaven rather than the rewards of earth.
Book of Mormon context: This is a lesson of particular importance to the Nephite people. The greed for temporal wealth had been at the heart of the Nehorite heresy as well as the most recent Gadianton influence. The temptation to the worldly wealth had been a great part of the changes that led to the destruction of the Nephite polity. The Nephites in Bountiful would not have been those who had accepted those temptations to worldly wealth, but they would certainly be in a position to appreciate this reinforcement of the nature of their relationship to wealth.
Textual: There are no changes from the Matthean text. In the 3 Nephi redaction as well as the Matthean text this verse begins a set of teachings that have no interrelationship. Whereas other sections of the sermon had thematic threads that linked the teachings, in this case we have various teachings that exemplify the gospel, but have no discernible relationship from one to the other.
The translation of this verse in the KJV uses “rust” as a translation for a word that more properly translations as “eating,” or perhaps an “eating-thing.” The literary parallel in the Greek would therefore be perhaps better translated as moth and worm rather than moth and rust. (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 326). The unstated treasure of the original would therefore be excess clothing and food rather than simply finery and metallic wealth. The rust would not apply to precious metals in any case. This concept of moth and worm would be a more applicable message for the New World. Nevertheless, the text was left with moth and rust.