Mormons believe that when Christ spoke of “other sheep” not of the Israelite fold, He was referring to those of the western-hemisphere civilization that produced the Book of Mormon. After His resurrection and ascension, Christ visited His people on the American continent, as He promised He would in the New Testament.
Mormons believe that the words of prophets on the American continent, contained in the Book of Mormon, serve as a second witness of Christ, in addition to the words of the biblical prophets.
2 Corinthians 13:1:
After describing the great apostasy, the Old-Testament prophet Isaiah prophesied of a sealed book that would be brought to one who is learned. The learned would not be able to read it, so the book would be delivered to one who was not learned. This prophesy was fulfilled in the 19th century. After the apostasy, God brought to light the Book of Mormon, a book that was delivered to the unlearned boy-prophet Joseph because the intellectuals of the time were too influenced by the “precept of man.”
Some conjecture that the Old-Testament prophet Ezekiel foresaw the coming forth of the Book of Mormon. The “stick,” or record, of Judah is the Bible. The stick of Ephraim may refer to the Book of Mormon, written by those of the descendents of Joseph that were led by God to the American continent.