How is it determined which texts are considered scripture and which are not? Specifically, why isn’t the King Follet Sermon given the status of scripture, since Joseph Smith stated some key doctrinal points in that sermon?
There are actually two classifications of scripture in the Church. One classification is the canonical scriptures. In other words, those scriptures that are accepted by thy body of the Church as being binding on church members. The canonical scriptures in The Church of Jesus Christ of Latter-day Saints are the Holy Bible, the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price.
The Church also accepts other writings and declarations as scripture, as declared in D&C 68:1-4—
My servant, Orson Hyde, was called by his ordination to proclaim the everlasting gospel, by the Spirit of the living God, from people to people, and from land to land, in the congregations of the wicked, in their synagogues, reasoning with and expounding all scriptures unto them.
And, behold, and lo, this is an ensample unto all those who were ordained unto this priesthood, whose mission is appointed unto them to go forth–
And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost.
And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.