The Lord operates by the system of witnesses. He will not destroy a people for wickedness until they have been adequately warned. He will not hold us responsible for rejecting his law unless we have been sufficiently taught his law. Therefore, in order to justify his punishment of the wicked, his word must also be established by more than one witness. Paul recited the ancient Hebrew law as follows, in the mouth of two or three witnesses shall every word be established (2 Cor 13:1). The word of God is no exception. In this verse we see that the Lord is declaring to the world that the Book of Mormon is the second witness, and is in fact, another testament of Jesus Christ. It was written with the intent to show the world that Jesus is the Christ, the Eternal God, manifesting himself unto all nations (Moroni’s Title Page). Elder Maxwell explains what the third witness is.
Neal A. Maxwell
“The splendid Book of Mormon advises that a third scriptural witness is yet to come from the lost tribes (see 2 Nephi 29:12-14)….We do not know when and how this will occur, but we are safe in assuming that the third book will have the same fundamental focus as the Book of Mormon—’that…their seed [too]…may be brought to a knowledge of me, their Redeemer’ (3 Nephi 16:4). If there is a title page in that third set of sacred records, it is not likely to differ in purpose from the title page in the Book of Mormon, except for its focus on still other peoples who likewise received a personal visit from the resurrected Jesus (see 3 Nephi 15:20-24; 16:1-4).” (First Nephi, The Doctrinal Foundation, BYU Religious Studies Center, p. 15 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 163)
Neal A. Maxwell
“Lost books are among the treasures yet to come forth. Over twenty of these are mentioned in the existing scriptures. Perhaps most startling and voluminous will be the records of the lost tribes of Israel (see 2 Nephi 29:13). We would not even know of the impending third witness for Christ except through the precious Book of Mormon, the second witness for Christ! This third set of sacred records will thus complete a triad of truth. Then, just as the Perfect Shepherd has said, ‘My word also shall be gathered in one’ (v. 14). There will be ‘one fold and one shepherd’ (1 Nephi 22:25 in a welding together of all the Christian dispensations of human history (see DC 128:18).” (Ensign, Nov. 1986, p. 52 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 163)