The Book of Mormon is the Keystone

President Ezra Taft Benson, who was our prophet during my younger years, taught that the Book of Mormon is the keystone of our religion in 3 ways: (1) Witness of Christ, (2) Fulness of doctrine, and (3) Foundation of testimony.

Witness of Christ
The Book of Mormon bears witness of the divinity of Jesus Christ- really that is its central purpose. It testifies of His reality with power and clarity. The Book of Mormon has helped me build a personal relationship with the Savior.

Fulness of Doctrine
The Book of Mormon contains the fulness of the doctrines required for our salvation, and it's written simple and plain enough so that even children can learn the pathway to salvation. The Lord said to the Prophet Joseph Smith that the Book of Mormon contains the "fulness of the gospel of Jesus Christ" in Doctrine and Convenants 20:9.

Foundation of Testimony
Any honest seeker of truth can read the Book of Mormon and pray to God to know if it is true. Praying with a sincere heart will lead to a divinely given testimony of the truth of this book. I know that this promise given in the Book of Mormon by the prophet Moroni is true because I've pray to God to find out if the Book of Mormon is true and he has testified to me on many occasions that it is by the power of the Holy Ghost. He has filled me with feelings of peace, joy, comfort, and love in answer to my prayers. By knowing that the Book of Mormon is true, I also know that the prophet who translated the book, Joseph Smith, must have been a true prophet of God. For a false prophet could never produce a true book (Remember Jesus taught that by their fruits, ye shall know them). The Book of Mormon is definitely a fruit of the prophet Joseph Smith. And through that testimony of the Book of Mormon, I also know that the Church of Jesus Christ of Latter-day Saints is the Kingdom of God, which has been re-established on the earth in these last days. And by the succession of prophets since Joseph Smith, I know that Thomas S Monson is a true prophet of God on the earth today.

This is why the Book of Mormon is the keystone.