Here Moroni instructs Joseph Smith concerning the calling of witnesses, chosen persons who will be, like the seer who brought forth the record, commissioned by God to bear solemn testimony of the truthfulness of the Book of Mormon.
“Wherefore, by the words of three, God hath said, I will establish my word’ (2 Nephi 11:3).Nephi had prophesied some 550 years before the coming of Christ: ”Wherefore, at that day when the book shall be delivered unto the man of whom I have spoken [Joseph Smith, the book [specifically the plates] shall be hid from the eyes of the world, that the eyes of none shall behold it save it be that three witnesses shall behold it, by the power of God, besides him to whom the book shall be delivered; and they shall testify to the truth of the book and the things therein’ (2 Nephi 27:12).
Oliver Cowdery, David Whitmer, and Martin Harris were each moved upon by the Spirit with a desire to become the witnesses so named in the Book of Mormon and inquired of the Prophet accordingly. The Lord confirmed that assignment upon them and thus their sacred witness is attached to every copy of the Book of Mormon (see D&C 17; The Testimony of Three Witnesses).