When I read the biography of Bruce McConkie by his son Joseph a few years back, I noticed that the book began by introducing Elder McConkie's roots. The author gives us details about Elder McConkie's parents, and his parents' parents. It's nice to know a little more about the main character of the book by tracing the line a bit. This is typical of most biographies that have been written about leaders of the Church. You're always going to get a little background about the family.
Nephi gives us the brief, but very clear genealogical background required for us to understand the identity and primary role of those who are called "Gentiles" throughout the entire book. The verses mentioned above are the beginning pieces of the picture. It is a picture that is developed fully only when considering the entirety of the Book of Mormon text.
Whether or not we can now accept that the Book of Mormon is a prophecy about and warning to us, we can begin by considering the resurrected Lord's teachings to Lehi's descendants who were spared destruction at the time of Christ's death. You can read the entire chapter for the context, but consider these words:
"But if the Gentiles will repent and return unto me, saith the Father, behold they shall be numbered among my people, O house of Israel.
"And I will not suffer my people, who are of the house of Israel, to go through among them, and tread them down, saith the Father.
"But if they will not turn unto me, and hearken unto my voice, I will suffer them, yea, I will suffer my people, O house of Israel, that they shall go through among them, and shall tread them down, and they shall be as salt that hath lost its savor, which is thenceforth good for nothing but to be cast out, and to be trodden under foot of my people, O house of Israel" (3 Ne. 16:13-15).
Skimming over these verses makes it easy to maintain a belief that the term Gentiles isn't talking about us. It is too easy for us to assume that when those heathen Gentiles repent they will then be numbered among the Church. That's not what this is talking about, however.
When in modern revelation the Lord repeats phrases he has previously used we should listen up. In our day the Lord has given us further light to understand the records we possess. On 16 December 1833 he revealed to the Prophet:
"When men are called unto mine everlasting gospel, and covenant with an everlasting covenant, they are accounted as the salt of the earth and the savor of men;
"They are called to be the savor of men; therefore, if that salt of the earth lose its savor, behold, it is thenceforth good for nothing only to be cast out and trodden under the feet of men" (D&C 101:39-40).
How can the Gentiles about whom Christ is prophesying in 3 Nephi "be as salt that hath lost its savor," if those Gentiles have not been "called" and have not made "covenants" by the proper authority? Those Gentiles who have never been "called," have thus never been "accounted" as the "salt of the earth and the savor of men." If the Gentiles about whom Christ is prophesying are indeed those who have been "called unto mine everlasting gospel," and who "covenant with an everlasting covenant" to obey it, how is it that they will, according to the words of Christ given unto Him by the Father, need to "repent and return unto me?"
When you consider the Book of Mormon in truth and with honesty, you begin to realize that you have many more questions than you thought you had answers. You need these questions. They lead you to Christ.
I've focused almost wholly on us, and the role we have to play, and our identity, and how the Book of Mormon is a warning to us. We should remember the book's preeminent purpose. It will show the remnant "what great things the Lord hath done for their fathers; and that they may know the covenants of the Lord, that they are not cast off forever" (Title Page). It was also given that the Gentiles might be convinced that "Jesus is the Christ" (ibid.). We can only fulfill our role when we properly understand it.