"If any man preaches to you, doctrines contrary to the Bible, the Book of Mormon, or the Book of Doctrine & Covenants, set him down as an imposter" (Joseph Smith, Times & Seasons, 5:490-491, April, 1, 1844.)
Joseph also said we could detect a bad angel if he “contradicted a former revelation” (TPJS p.215)
In His great love, He did not leave us alone. He provided a guide—a spiritual road map—to help us achieve success in our journey. We call that guide the standard works, so named because they—the Holy Bible, the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price—constitute the standard by which we should live. They serve as a standard of reference, as are standards of time, weights, and measures that are kept in national bureaus of standards. (Russell M. Nelson – October 2000)
The Standard Works ... provide the standard by which all gospel doctrine is measured. All other [materials] should spring from the word of the Lord as set forth in these volumes ("Cornerstones of Responsibility," Regional Representatives' Seminar, 5 Apr. 1991, 2).
I have spoken before about the importance of keeping the doctrine of the Church pure, and seeing that it is taught in all of our meetings. I worry about this. Small aberrations in doctrinal teaching can lead to large and evil falsehoods.... When all is said and done, the test of the doctrine lies in the standard works of the Church. These have been accepted in conference and assembled as our doctrinal standards. (General Authority Training Meeting, October 1, 1996. Cited in Teachings of Gordon B. Hinckley, 620, 574).
And if any man speak a doctrine which contradicts what is in the standard Church works, you may know by that same token that it is false and you are not bound to accept it as truth. (The First Area General Conference for Germany, Austria, Holland, Italy, Switzerland, France, Belgium, and Spain of the Church of Jesus Christ of Latter-day Saints, held in Munich Germany, August 24–26, 1973, Reports and Discourses, p.69)
“‘The Standard Works’ … are the reservoir of our doctrine from which flows the waters of gospel light. They provide the standard by which all gospel doctrine is measured. All other [materials] should spring from the word of the Lord as set forth in these volumes” (“Cornerstones of Responsibility” [regional representatives’ seminar, 5 Apr. 1991], 2).