There are fourteen references to the “fulness of the gospel” in the Doctrine and Covenants. Of these, four of them specifically mention that the Book of Mormon contains this fulness (D&C 20:9; 27:5; 42:12; 135:3).
In response to the question as to what constitutes the fulness, President Joseph Fielding Smith said that the Lord “did not mean to convey the impression that every truth belonging to exaltation in the kingdom of God was recorded in the Book of Mormon. … Neither would this statement imply that every truth belonging to the celestial kingdom and exaltation therein was to be found within the covers of the Book of Mormon. There are many truths belonging to the exaltation that have not been revealed, nor will they be revealed to man while he is in mortality. …
”It is evident that there are many things that belong to the exaltation which are reserved for immortal glorified souls. The fulness of the gospel then, as expressed in the Doctrine and Covenants, has reference to the principles of salvation by which we attain unto this glory. Therefore, the Lord has revealed in the Book of Mormon all that is needful to direct people who are willing to hearken to its precepts, to a fulness of the blessings of the kingdom of God. The Book of Mormon then, does contain all the truths which are essential for Gentiles and Jews or any other people, to prepare them for this glorious exaltation in the celestial kingdom of God” (AGQ, 3:95-96; italics added).
The Lord Himself described the fulness of the gospel as “the covenant which I have sent forth to recover my people, which are of the house of Israel” (D&C 39:11). This recovery occurs as men, women, and children of the age of accountability accept the gospel and comply with its ordinances (A of F 1:4-5). President Harold B. Lee identified Doctrine and Covenants 39:6 as a “one-sentence” statement which defines the “fulness of the gospel” (SHP, 50-51).