Paul opens his argument with the example of marriage as a permanent union, symbolizing the permanence of the Mosaic law. Marriage is a binding relationship for a woman as long as the husband lives. Then the widow is no longer married and may marry another.
Paul sees the Torah as a binding relationship between the covenant people and God only until the Torah becomes “dead.” At that point, the spouse is freed from the binding relationship and is able to enter into another union. In this analogy, Paul creates a division between the Torah and a Christianity not bound by the Torah. If the passage in Nephi had been informed by Paul’s meaning, rather than only Paul’s language, we would expect Nephi to also deny the need for circumcision or any other form of Torah observance. Not only is this reaction absent, but Nephi has the exactly opposite reaction. Nephi’s people continue their relationship to the Torah, adding on the realization that it is the Messiah who will save, not the Mosaic law. Therefore, because the Torah does not save, it is “dead.”
In translating the Book of Mormon, Joseph Smith used the religious vocabulary he had already learned to develop certain themes. In this case, because of Joseph’s familiarity with Paul, he uses Paul’s vocabulary to describe Nephi’s intent. That intent contrasts completely, however, despite the similar vocabulary.