In the Doctrine and Covenants, section 20, verses 8-10, the Lord has said that the Book of Mormon contains “the fulness of the gospel of Jesus Christ to the Gentiles and to the Jews also.” Many people have raised the question as to how the Book of Mormon could actually contain the fulness of the gospel when it doesn’t even refer to such important ordinances as the temple endowment and marriage for eternity. Concerning this question, Joseph Fielding Smith has written:
First of all, let us consider what the Lord means by “a fulness of the gospel.” He did not mean to convey the impression that every truth belonging to exaltation in the kingdom of God had been delivered to the Nephites and was recorded in the Book of Mormon, to be delivered to Gentiles and Jews in this dispensation. 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. We must concede it to be a fact that there are many things related to the exaltation which cannot be received now and do not concern mortal men. These truths were not given to the Nephites; neither can they be given to us in this present day, for they do not in any way apply to the needs of the mortal condition, nor could we comprehend them while we are in mortality. These things belong to the kingdom of God and will be revealed to those who attain to the celestial exaltation.
… 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.
It is beyond dispute, or should be, that the Book of Mormon teaches that the first principles of the gospel are, faith in God; repentance from sin; baptism for the remission of sins; the gift of the Holy Ghost, obedience to divine law and that man cannot be saved in ignorance of these divine truths. It teaches that “wickedness never was happiness,” and that no man can be saved without repentance of sin. (Answers to Gospel Questions, 3:95-97.)
President Charles W. Penrose has also written on this subject:
Now, some of our brethren have taken up quite a discussion as to the fulness of the everlasting gospel. We are told that the Book of Mormon contains the fulness of the gospel, that those who like to get up a dispute, say that the Book of Mormon does not contain any reference to the work of salvation for the dead, and there are many other things pertaining to the gospel that are not developed in that book, and yet we are told that book contains “the fulness of the everlasting gospel.” Well, what is the fulness of the gospel? You read carefully the revelation in regard to the three glories, section 76, in the Doctrine and Covenants, and you find there defined what the gospel is. There God, the Eternal Father, and Jesus Christ, His Son, and the Holy Ghost, are held up as the three persons in the Trinity—the one God, the Father, the Word, and the Holy Ghost, all three being united and being one God. When people believe in that doctrine and obey the ordinances which are spoken of in the same list of principles, you get the fulness of the gospel for this reason: If you really believe so as to have faith in our Eternal Father and in his Son, Jesus Christ, the Redeemer, and will hear Him, you will learn all about what is needed to be done for the salvation of the living and the redemption of the dead.
When people believe and repent and are baptized by Divine authority, and the Holy Ghost is conferred upon them as a gift, they receive the everlasting gospel. We used to call it, and it is now called in the revelations, the “Gift of the Holy Ghost,” the Holy Ghost, the spirit that proceeds from the presence of the Father throughout the immensity of space, which guides, directs, enlightens, which is light in and of itself, which is the spirit of intelligence, the “light of truth… .” (Conference Report, April 1922, pp. 27-28.)