Latter-day Saints don’t hold to any creed or single definitive statement of beliefs. Joseph Smith once declared, “The fundamental principles of our religion are the testimony of the Apostles and Prophets, concerning Jesus Christ, that He died, was buried, and rose again the third day, and ascended into heaven; and all other things which pertain to our religion are only appendages to it” (source).
That being said, the most obvious answer for who sets doctrine for Mormons is the President of the Church, colloquially called the Prophet (though technically, the top fifteen leaders – three in the First Presidency and twelve Apostles – are all considered prophets). Sermons given to the worldwide church at the twice-annual General Conference by these fifteen leaders are considered authoritative by Church members and can set or clarify doctrine, with the President’s statements taking precedence. So Church leaders, especially the President, set the doctrine.
However, Mormons don’t believe in prophetic infallibility and there have certainly been times when a current president or leader has contradicted previous presidents or other leaders. When that happens, in line with the LDS belief in continuing revelation, current statements generally take precedence over those of prior prophets. So current Church leaders set the doctrine.
Lesson manuals and resource books officially published by the Church go through an extensive editing and correlation process via various committees which are overseen by those fifteen leaders to ensure that statements are doctrinally correct and consistent. But they are not considered “doctrine” themselves. Likewise, books written by Church leaders and published by a press other than the official Church press all have disclaimers printed in the front or the forward that the books are not declarations of official Church doctrine and the individual authors are solely responsible for the contents. Some members still consider such books authoritative because of the author’s position in the Church, but they are not considered “doctrine”.
Church leaders have also defined “official doctrine” as that which is found in the “standard works” of scripture that Mormons use: the Holy Bible, the Book of Mormon, the Doctrine & Covenants, and the Pearl of Great Price (which includes the Articles of Faith, the closest thing we have to a creed). One of the previous Presidents of the Church, Joseph Fielding Smith, stated that every statement of doctrine by Church leaders should be judged against the doctrine found in the standard works:
It makes no difference what is written or what anyone has said, if what has been said is in conflict with what the Lord has revealed, we can set it aside. My words, and the teachings of any other member of the Church, high or low, if they do not square with the revelations, we need not accept them. Let us have this matter clear. We have accepted the four standard works as the measuring yardsticks, or balances, by which we measure every man’s doctrine. You cannot accept the books written by the authorities of the Church as standards in doctrine, only in so far as they accord with the revealed word in the standard works.(Source)
There is a process whereby the Church as a whole can vote to approve new scripture and add to the standard works (which has happened only a handful of times since the Church’s founding in 1830, most recently in 1978). So scripture, recognized and canonized by the Church as a whole, sets the doctrine.
There are, of course, tons of different interpretations of scripture, so scripture and Church leaders’ statements together elucidate and clarify LDS doctrine. Mormons are also strongly encouraged to seek for and receive personal confirmation from the Spirit of the doctrines taught, so in practice those three sources (the “standard works” scriptures, Church leaders’ statements, and the Spirit) work together to set/confirm Mormon doctrine.
A few years back, the Church issued an official statement about doctrine, which reads in part:
Not every statement made by a Church leader, past or present, necessarily constitutes doctrine. A single statement made by a single leader on a single occasion often represents a personal, though well-considered, opinion, but is not meant to be officially binding for the whole Church. With divine inspiration, the First Presidency (the prophet and his two counselors) and the Quorum of the Twelve Apostles (the second-highest governing body of the Church) counsel together to establish doctrine that is consistently proclaimed in official Church publications. This doctrine resides in the four “standard works” of scripture (the Holy Bible, the Book of Mormon, the Doctrine and Covenants and the Pearl of Great Price), official declarations and proclamations, and the Articles of Faith.
Emily H. Geddes was born to two physicists and grew up as a Navy brat. Born-and-raised as a member of The Church of Jesus Christ of Latter-day Saints, she holds a bachelor's degree in theatre from Brigham Young University, and earned an MBA from Eastern Washington University.