The President of the Church, not any one Apostle, determines official LDS theology. Newly formed official LDS doctrine is put forth by the President of the Church in an official statement countersigned either by his counselors or all members of the Quorum of the Twelve Apostles, or both. No one man posits new doctrine alone.
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.
Isolated statements are often taken out of context, leaving their original meaning distorted.
Some doctrines are more important than others and might be considered core doctrines. For example, the precise location of the Garden of Eden is far less important than doctrine about Jesus Christ and His atoning sacrifice.
The mistake that public commentators often make is taking an obscure teaching that is peripheral to the Church’s purpose and placing it at the very center. This is especially common among reporters or researchers who rely on how other Christians interpret Latter-day Saint doctrine.
Because different times present different challenges, modern-day prophets receive revelation relevant to the circumstances of their day. This follows the biblical pattern (Amos 3:7), in which God communicated messages and warnings to His people through Prophets in order to secure their well-being. Based on the Scriptures, Joseph Smith 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.”
http://www.mormonnewsroom.org/ldsnew...ormon-doctrine