B. H. Roberts testifies that the D&C and POGP are evidence for Joseph's inspiration like the Book of Mormon is.
B. H. Roberts, Speech, April 1929, in Ninety-Ninth Annual Conference of the Church of Jesus Christ of Latter-day Saints (Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1929), 118–121
One of the things that has greatly delighted me in this conference has been the prominence given to the Book of Mormon and to the importance of it as a means of acquainting the world with that system of truth for which we stand. But the passage from our articles of faith just repeated reminds me that the Book of Mormon is only one out of very many things that may aid us in this work of making God's message known to the world.
We seldom hear the Doctrine and Covenants spoken of as a volume of scripture and important as a help in convincing the world of the truth of our message. Perhaps I can present the thought I hold in my mind upon that subject by relating a circumstance that happened very many years ago in the Southern States. On one of the branch streams of the Tennessee river in one of our conference districts, there lived a woman of some considerable local fame, I may say, noted for her strong character, her intelligence and her religious sincerity. It so happened that she invited us to her home on one of our visits to induce her to read the Book of Mormon and to pay attention to the doctrines we had explained. Several of the local ministers who rather depended upon her as something of a pillar in one of their churches, heard with alarm the fact that she was reading the Book of Mormon, and called upon her to persuade her to give up her perusal of it ; and gave her the stereotyped idea used by opponents of the book at that time, and brought to her pamphlets and articles from periodicals to show that the Book of Mormon was fiction and originated in the Spaulding Romance. They urged her to read this testimony against the book, and she promised them she would do so. In the course of a week or two they returned to her to inquire the progress she was making, and she answered them in substance in this way: "I am somewhat confused in relation to this Book of Mormon. The Mormon elders tell one story of its origin and you tell another, and I must confess I am somewhat perplexed about it; but," said she, "here is another book that the Mormon elders have presented to me and which I have read. They call it the Doctrine and Covenants. It purports to contain a number of revelations to Joseph Smith which he is said to have received. It is nearly equal in volume to the Book of Mormon, and there is no question at all in relation to the authorship of this book. None of you questions, that Joseph Smith wrote it. He is the author of it, and claims everything in it to be inspired of God; and I wish to state to you," said she, "that this book, — the revelations that are in it — contains as much evidence, and even more evidence, that the man who produced it was inspired of God than does the Book of Mormon that it was written by inspiration. Now what have you to say to that, and how will you explain away that?" Of course they had no explanation. The Book of Doctrine and Covenants stands unquestioned as to its authorship, and I wish to express a belief that there is evidence of inspiration in it equal to that of the Book of Mormon.
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Perhaps I might call your attention to the fourth book of scripture which the Church accepts officially and by which it is willing to be judged, — The Pearl of Great Price. The Bible, the Book of Mormon, the Doctrine and Covenants and the Pearl of Great Price are prized by us above all other books. If the world but had the Pearl of Great Price, and the knowledge it conveys, it would shed a penetrating light upon all the scriptures that our Christian friends acknowledge, and make known the truth of God; how, from the beginning, instead of dealing with mere fragments and hints at the plan of salvation, it would set forth the whole plan clearly. About all the world has in the Old Testament about the Gospel is the statement in Genesis that the seed of the woman should bruise the serpent's head — wound him in a vital part, overcome him; and the implication that is to. be seen in the offering of Abel, and the rejection of the offering by Cain; with here and there an indication of information that underlaid the testimony of those patriarchs that they had some knowledge of the Gospel of Jesus Christ. But from the book of Moses and in the Book of Abraham, in the Pearl of Great Price, we get revelation of the plan of salvation that was instituted in heaven among the spirits that were the children of God, and an explanation of the earth life of man that is truly enlightening. Patriarchs from Adam to Noah have been some of the mightiest servants of God and the greatest prophets. They knew very much more than the fragmentary knowledge that has come to the world in our day.
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This book of Scripture, the Book of Moses, as well as the Book of Mormon, then, brings light and truth into the world for the salvation of men. God grant that this light and truth may be extended among the nations, is my prayer in the name of Jesus Christ. Amen.