Summary: Three witnesses and the work itself shall stand as a testimony of the truthfulness of the Book of Mormon.
This chapter is only a few verses long, but it is succinct and important. Moroni is addressing the future translator of the plates, who we know to be Joseph Smith. It is interesting that he would write directly and specifically to him when almost 14 centuries elapse from the time this is written from the time Joseph even reads the words.
The institute manual points out something interesting in verse 4:
And in the mouth of three witnesses shall these things be established; and the testimony of three, and this work, in the which shall be shown forth the power of God and also his word, of which the Father, and the Son, and the Holy Ghost bear record—and all this shall stand as a testimony against the world at the last day.
We understand the three witnesses to be Martin Harris, Oliver Cowdery, and David Whitmer. (read their testimony here). However, the Institute Manual adds that the Godhead are also witnesses of the Book of Mormon.
Dallin H. Oaks, a living apostle (also having vast experience in law), emphasizes the importance of witnesses. This quote is lengthy but good:
“Persons experienced in evaluating testimony commonly consider a witness’s opportunity to observe an event and the possibility of his bias on the subject. Where different witnesses give identical testimony about the same event, skeptics look for evidence of collusion among them or for other witnesses who could contradict them.
“Measured against all of these possible objections, the testimony of the Three Witnesses to the Book of Mormon stands forth in great strength. Each of the three had ample reason and opportunity to renounce his testimony if it had been false or to equivocate on details if any had been inaccurate. As is well known, because of disagreements or jealousies involving other leaders of the Church, each one of these three witnesses was excommunicated from The Church of
Jesus Christ of Latter-day Saints by about eight years after the publication of their testimony. All three went their separate ways, with no common interest to support a collusive effort. Yet to the end of their lives—periods ranging from 12 to 50 years after their excommunications—not one of these witnesses deviated from his published testimony or said anything that cast any shadow on its truthfulness.
“Furthermore, their testimony stands uncontradicted by any other witnesses. Reject it one may, but how does one explain three men of good character uniting and persisting in this published testimony to the end of their lives in the face of great ridicule and other personal disadvantage? Like the Book of Mormon itself, there is no better explanation than is given in the testimony itself, the solemn statement of good and honest men who told what they saw. . . .
“. . . Witnesses are important, and the testimony of the Three Witnesses to the Book of Mormon is impressive and reliable” (in Conference Report, Apr. 1999, 46, 49; or Ensign, May 1999, 35–37).
I also add my testimony--that I know this book is true. I know the prophets and the people written about in the Book of Mormon really lived. I have prayed about this book and had an immediate, powerful verification of the truthfulness and importance of this book. I cannot deny the feelings I have had and the ways this book has improved my life. The fruits of this book are good.
We can spend our time proving the logical or historical accuracy of this book, but we cannot ignore the most important part: it is written by prophets of God who bear testimony of Christ and instruct us on how to know and emulate Him better. It is a message of hope. Without these things, it would remain, simply, a history book. I know it is much more than that. If you find yourself doubting its truthfulness, I suggest that you read it and pray about it. Heavenly Father is mindful of your desires and will answer your prayer.