D&C 17

Moses taught in his law -

"...at the mouth of three witnesses, shall the matter be established."  (Old Testament  Deuteronomy 19:15)

Following that, the Savior continue to teach -

"...that in the mouth of two or three witnesses every word may be established."  (New Testament  Matthew 18:16)

As we read the Book of Mormon, ancient prophets prophesied concerning the coming forth of their writings -

"...behold, ye may be privileged that ye may show the plates unto those who shall assist to bring forth this work;
3 And unto three shall they be shown by the power of God; wherefore they shall know of a surety that these things are true.
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."  (Book of Mormon   Ether 5:2 - 4)

And -

"...at that day when the book shall be delivered unto the man of whom I have spoken, the book 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."  (Book of Mormon  2 Nephi 27:12)

It  note that in both these prophesies, that witnesses would not only see but they would witness by the power of God.  As the Prophet Joseph Smith met with Oliver Cowdery, David Whitmer and Martin Harris, the Lord revealed -

"BEHOLD, I say unto you, that you must rely upon my word, which if you do with full purpose of heart, you shall have a view of the plates, and also of the breastplate, the sword of Laban, the Urim and Thummim, which were given to the brother of Jared upon the mount, when he talked with the Lord face to face, and the miraculous directors which were given to Lehi while in the wilderness, on the borders of the Red Sea.
2 And it is by your faith that you shall obtain a view of them, even by that faith which was had by the prophets of old.
3 And after that you have obtained faith, and have seen them with your eyes, you shall testify of them, by the power of God..."  (Doctrine and Covenants  Section 17:1 - 3)

Of which they did as instructed.  Such testimony is provided in each copy of the Book of Mormon.  The Lord then tells them why -

"...this you shall do that my servant Joseph Smith, Jun., may not be destroyed, that I may bring about my righteous purposes unto the children of men in this work."  (Doctrine and Covenants   Section 17:4)

I was touched by the words of Lucy Mack Smith as she records -

“In a few minutes after this, Joseph, Martin, Oliver and David, repaired to a grove, a short distance from the house, where they commenced calling upon the Lord, and continued in earnest supplication, until he permitted an angel to come down from his presence, and declare to them, that all which Joseph had testified of concerning the plates was true.

 “When they returned to the house it was between three and four o’clock p.m. (David Whitmer indicated that they left the house about 11 a.m. Therefore, it appears that they were gone from the house between four and five hours.) Mrs. Whitmer, Mr. Smith and myself, were sitting in a bedroom at the time. On coming in, Joseph threw himself down beside me, and exclaimed, ‘Father, mother, you do not know how happy I am: the Lord has now caused the plates to be shown to three more besides myself. They have seen an angel, who has testified to them, and they will have to bear witness to the truth of what I have said, for now they know for themselves, that I do not go about to deceive the people, and I feel as if I was relieved of a burden which was almost too heavy for me to bear, and it rejoices my soul, that I am not any longer to be entirely alone in the world.’ Upon this, Martin Harris came in: he seemed almost overcome with joy, and testified boldly to what he had both seen and heard. And so did David and Oliver, adding that no tongue could express the joy of their hearts, and the greatness of the things which they had both seen and heard.” (Lucy Mack Smith, History of Joseph Smith by His Mother [Salt Lake City: Stevens & Wallis, Inc., 1945], 151.)

It is touching to sense the great burden that Joseph had experience and the relief he felt as he knew others had the same witness.