Laman and Lemuel ask Nephi an odd question after he concludes his reading of Isaiah. It echos a previous question they asked him as he taught them about their father's vision of the tree of life - note that we don't have many of Laman and Lemuel's words to provide an insight into their logic, so any repetition is revealing - besides the usual anger, murmuring, and attempted murdering.
And here is the previous question from First Nephi 15: 30-32
It is safe to say Laman and Lemuel had a tendency want to separate the temporal from the spiritual. Whether this is a personal predilection or a doctrine commonly taught among those at Jerusalem, it doesn't really matter, it is incorrect. Not only do their questions miss the point, in someway they seem to reveal a method of justification Laman and Lemuel might have employed: if spiritual and temporal are separate, perhaps what we do temporally is less important.
Nephi teaches that the gospel, the plan of salvation, and God's commandments encompass things both temporal and spiritual. This would be further clarified by the Lord centuries later in D&C 29:34-35:
Wherefore, verily I say unto you that all things unto me are spiritual, and not at any time have I given unto you a law which was temporal; neither any man, nor the children of men; neither Adam, your father, whom I created ... My commandments are spiritual; they are not natural nor temporal, neither carnal nor sensual.
Both of these examples are also occurrences of Nephi being a teacher to his brethren, in fulfillment of the Lord's promise to him in First Nephi 2:22.