The testimony of the three witnesses, which appear at the front of the Book of Mormon (Oliver Cowdery, David Whitmer, and Martin Harris) declares 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 engraving thereon.
It is quite noteworthy that Martin Harris denied that he had actually seen the plates with his naked eyes. In fact, when pressed, he stated, No, I saw them with a spiritual eye (Recollections of John H. Gilbert, 1892, Typescript, BYU, 56).
The Mormons are loath to admit that all three of these witnesses later apostatized from the Mormon faith and were described in most unflattering terms (counterfeiters, thieves, [and] liars) by their Mormon contemporaries (cf. Senate Document 189, February 15, 1841, 69).
A careful check of early Mormon literature also reveals that Joseph Smith wrote prophecies and articles against the character of the witnesses of the Book of Mormon, which in itself renders their testimony suspect (cf. Doctrine and Covenants, 3:12; 10:7; History of the Church; 3:228, 3:232).
Mormons try to cover this historical predicament by saying that two of the three witnesses, Oliver Cowdery and Martin Harris, were rebaptized into Mormonism. What they fail to reveal is more significant: The Times and Seasons (2:482) published that Oliver Cowdery denied his Book of Mormon testimony. He spent several years as a baptized Methodist before his rebaptism into Mormonism. Martin Harris, likewise, has suspicious circumstances surrounding his rebaptism. He denied the teachings of Brigham Young after rebaptism and was banned from preaching by Young because of their differences. David Whitmer changed the details of his testimony concerning the angel with the golden plates to say that it was a vision and not an actual visitation by an angelic person (An Address to All Believers in Christ, p. 32). Certainly testimony from such unstable personalities is dubious at best.
Source Walter Martins "Kingdom of the Cults"