David Whitmer was privileged to be one of the three witnesses of The Gold Plates. He was one of the first supporters of Joseph Smith he was able to see and experience many miracles. Even though David did leave the Church, he never denied what he saw and experienced.
David Whitmer first heard of Joseph Smith and the Gold Plates while on a business trip to Palmyra, New York. David dismissed the rumors he heard of Joseph until he met Oliver Cowdery, who believed there was some truth to the rumors. David met the prophet Joseph Smith in 1829, through Oliver. They met while Joseph Smith was translating The Book of Mormon.
David had an experience of angelic help clearing his fields that helped convince him of the Lords desire for him with the restoration of the gospel.(Read more here).
Joseph Smith faced persecution in Harmony Pennsylvania while translating The Book of Mormon. David Whitmer volunteered his family home in Fayette New York for the finishing of the translation of The Book of Mormon and The Church of Jesus Christ was organized at his family home.
David readily accepted the gospel message and was one of the three witnesses to see the Golden Plates. In his own words he says: “a dazzlingly brilliant light that surpassed in the brightness even the sun at noonday and . . . A personage clothed in white and near him a table containing the ancient artifacts.”
David Whitmer was one of the first people to be baptized in this dispensation, he is numbered among the first six original members of the restored gospel.
David followed with the members to Kirtland Ohio and then on to Missouri where he became upset at the prophet and left The Church.
David Whitmer never went back or wavered on his Testimony of the golden plates. He was widely known as “the last surviving witness.” He said "thousands came to inquire if it was true."
Just before his death David called his family around him and bore one more witness of what he saw. He said “Dr. Buchanan, I want you to say whether or not I am in my right mind, before I give my dying testimony.” The doctor answered, “Yes, you are in your right mind.” David, then, bore testimony: “I want to say to you all, the Bible the record of the Nephites(Book of Mormon) is true, so you can say that you have heard me bear testimony on my death-bed.”
References:
”Who’s Who in the Doctrine and Covenants”(p.328-330)
https://history.churchofjesuschrist.org/content/museum/museum-treasures-david-whitmer-trunk?lang=eng