In verse 28 Nephi declared that “many plain and precious things [were] taken away from the book.” This verse (along with vv. 35–36) indicate that the Book of Mormon is a record that is to be restored in the last days. The purpose of the Book of Mormon is to restore lost plain and precious parts. They are not part of a new gospel.
In the textual history of the Bible, we may have lost entire books. We have likely lost paragraphs, sentences, and words. Some things have been “lost” because words or sentences were added to the Bible. All of those things changed the Bible in some way, but not in the way that Nephi intends when he suggests that the plain and precious things have been lost. What was lost is that which can be found in the Book of Mormon: a firm declaration that Jesus is the Christ.
Variant: The printer’s manuscript and the 1830 edition read: “the Lamb of God is the Eternal Father.” This was corrected to “the Lamb of God is the son of the Eternal Father” beginning in the 1837 edition and continuing to the present. (See commentary accompanying 1 Nephi 11:18 and “Excursus: The Nephite Understanding of God,” following 1 Nephi 11.)