The plates had to be shown to the three witnesses by the power of God. This means that Joseph Smith could not just show them the plates and ask them to testify. This would not do. Others would contend that he had fashioned some ancient appearing record in order to dupe the three witnesses into a false testimony. In order to prove to the world that Joseph Smith was not a magician and the greatest deceiver of his day, the Lord would reveal the truth of the Book of Mormon to the three witnesses by the indisputable power of God.
“It has been the boast of so-called sleight-of-hand performers that they can duplicate every trick of spiritists, no matter how astounding; but here is a plain divine manifestation of which no human art, or ingenuity, can furnish an imitation.” (Reynolds and Sjodahl, Commentary on the Book of Mormon, 6:111)
Accordingly, the three witnesses were not just shown the plates. Instead, they were shown a vision from God in which they saw an angel, saw a light from heaven, and heard a voice. Appropriately, Moroni himself appeared to them and showed them the gold plates, the sword of Laban, the Liahona, the breastplate, and the Liahona (see commentary on the three witnesses and DC 17:1).
Joseph Smith
“…we beheld a light above us in the air, of exceeding brightness; and behold, an angel stood before us. In his hands he held the plates which we had been praying for these to have a view of. He turned over the leaves one by one, so that we could see them, and discern the engravings thereon distinctly. He then addressed himself to David Whitmer, and said, ’David, blessed is the Lord, and he that keeps His commandments;’ when, immediately afterwards, we heard a voice from out of the bright light above us, saying, ’These plates have been revealed by the power of God, and they have been translated by the power of God. The translation of them which you have seen is correct, and I command you to bear record of what you now see and hear.’” (History of the Church, 1:54-55)
Testimony of the Three Witnesses
“…we…have seen the plates…And we also know that they have been translated by the gift and power of God, for his voice hath declared it unto us; wherefore we know of a surety that the work is true. And we also testify that we have seen the engravings which are upon the plates; and they have been shown unto us by the power of God, and not of man. And we declare with words of soberness, that an angel of God came down from heaven, and he brought and laid before our eyes, that we beheld and saw the plates, and the engravings thereon; and we know that it is by the grace of God the Father, and our Lord Jesus Christ, that we beheld and bear record that these things are true. And it is marvelous in our eyes…”
Orson Pratt
“…the Lord confirmed the truth of the Book of Mormon unto many witnesses in such a way, and by such means, that it was impossible for them to have been deceived;…the testimony of these witnesses stands good until it can be refuted; [and] no man can be justified in rejecting this testimony until he can show that it is false…Now, no one has ever attempted to bring any direct negative testimony; this, indeed, would be impossible, unless the witnesses themselves should deny their former testimony, and this they have not done. And those who have attempted to condemn their testimony from the nature of the message itself, have only exhibited their own weakness and folly. Upwards of twenty years (now 170 yrs.) have passed away, and no man has, as yet, been found able to prove the Book of Mormon or the testimony of its witnesses false.” (Pamphlet: “Divine Authenticity of the Book of Mormon,” p. 65)