Again, this message was written specifically for Joseph Smith. When the prophet translated this portion of the record, he was prompted to show the plates unto three witnesses. This was according to the promise which had been previously given to the prophet, in addition to your testimony, the testimony of three of my servants…shall go forth with my words that are given through you. Yea, they shall know of a surety that these things are true, for from heaven will I declare it unto them (DC 5:11-12). However, the demonstration of the record was to be done according to the will of the Lord, for the record was to be shown by the power of God (v. 3).
Joseph Smith
"In the course of the work of translation, we ascertained that three special witnesses were to be provided by the Lord, to whom He would grant that they should see the plates from which this work (the Book of Mormon) should be translated; and that these witnesses should bear record of the same, as will be found recorded, Book of Mormon, page 581 [Book of Ether, chapter 5, verses 2, 3 and 4, p. 487, edition 1920], also page 86 [II Nephi, chapter 11, verse 3, p. 73, edition 1920]. Almost immediately after we had made this discovery, it occurred to Oliver Cowdery, David Whitmer and the aforementioned Martin Harris (who had come to inquire after our progress in the work) that they would have me inquire of the Lord to know if they might not obtain of him the privilege to be these three special witnesses; and finally they became so very solicitous, and urged me so much to inquire that at length I complied; and through the Urim and Thummim, I obtained of the Lord for them the following: (DC 17)
’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 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.
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.
And after that you have obtained faith, and have seen them with your eyes, you shall testify of them, by the power of God.’"
(History of the Church, 1:52-53, see also DC 17:1-3)