Wilford Woodruff summarized a day he spent with Joseph Smith (28 November 1841) by writing in his journal that "Joseph said the Book of Mormon was the most correct of any Book on earth & the keystone of our religion & a man would get nearer to God by abiding by its precepts than any other Book."
Whether this was a verbatim quotation or Woodruff's summary, the keystone framing has endured. President Ezra Taft Benson observed
A keystone is the central stone in an arch. It holds all the other stones in place, and if removed, the arch crumbles.
There are three ways in which the Book of Mormon is the keystone of our religion. It is the keystone in our witness of Christ. It is the keystone of our doctrine. It is the keystone of testimony.
(1986, October, Ezra Taft Benson, ‘The Book of Mormon—Keystone of Our Religion,’ Ensign, November 1986, ¶ 15–16)