From the beginning of 1 Nephi to 2 Nephi 5, Nephi wrote of sacred things as couched in the history of the origin of the Nephites as a new people. He included a sermon from his brother that began to shift the focus from history to theology. At this point, he begins to explain why the long passages from Isaiah have been included. They are relevant to the reason he is currently writing. It appears that he finished his discussion of sacred history. He wrote all that had been included in the header for the book of 2 Nephi.
At this point, he writes “unto my people, unto all those that shall receive hereafter these things which I shall write.” What Nephi will do from now to the end of 2 Nephi, which is the end of his personal writing, is describe how the words or Isaiah underscore the message Jehovah declares to all nations.
Nephi also speaks of plainness. For Nephi, this means an understanding that is informed by the spirit of prophecy. Understanding prophecy requires a connection to the source of that prophecy. Thus, Nephi will speak in plainness because he has such a connection to the author of prophecy. He declares that he does this that his people, and future readers, may learn what they might not otherwise have understood.