And gave him (Joseph Smith) power from on high, by the means which were before prepared,

to translate the Book of Mormon;

Which contains a record of a fallen people,

and the fulness of the gospel of Jesus Christ to the Gentiles and to the Jews also...

(D&C 20:8-9)

The word 'fulness' is used numerous times in the scriptures; used to signify 'completeness,' 'all that is therein' and 'full.' as identified in Strong's Concordance. The Lord, for example, in Psalms 16:11 states:

“Thou wilt shew me the path of life: in thy presence is fulness of joy; at thy right hand there are pleasures for evermore.”

representing the idea that the object could not receive any more joy. The less pleasant use of the word can be found in 1 Nephi 17:35:

"...But behold, this people had rejected every word of God, and they were ripe in iniquity; and the fulness of the wrath of God was upon them; and the Lord did curse the land against them..."

It is easily conceived that the 'fulness of the wrath of God' doesn't leave much room in the 'glass.' The word is also used to signify that a period is complete as is found in Ephesians 1:10:

"That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him."

The same can be said by the attribution to the gospel. The fulness of the gospel, as cited in the above scripture in Section 20, represents all that is contained in the gospel. As such, the Book of Mormon contains all that is the gospel of Jesus Christ. This idea is reinforced in 1 Nephi 15:13 which reads:

“And now, the thing which our father meaneth concerning the grafting in of the natural branches through the fulness of the Gentiles, is, that in the latter days, when our seed shall have dwindled in unbelief, yea, for the space of many years, and many generations after the Messiah shall be manifested in body unto the children of men, then shall the fulness of the gospel of the Messiah come unto the Gentiles, and from the Gentiles unto the remnant of our seed—“

As can be seen by the above references, both the Doctrine and Covenants and the Book of Mormon represent that the fulness of the gospel is contained in the Book of Mormon. The Joseph Smith History found in the Pearl of Great Price helps to zero in on the location of this information speaking of Moroni's message to Joseph Smith in verse 34:

"He said there was a book deposited, written upon gold plates, giving an account of the former inhabitants of this continent, and the source from whence they sprang. He also said that the fulness of the everlasting Gospel was contained in it, as delivered by the Savior to the ancient inhabitants;"

The visit of Jesus Christ to the 'ancient inhabitants' is found in 3rd Nephi; specifically between his descent from heaven in Chapters 9-11 and his subsequent departure found in Chapter 28 verse12. It is made even easier since, rather than leaving it to speculation, the Lord was very specific on this topic. As with any good speaker, the Lord told us what he was going to tell us, told us, and then told us what he had told us. This is found in 3rd Nephi, Chapter 27 starting with verse 13:

"13 Behold I have given unto you my gospel, and this is the gospel which I have given unto you—that I came into the world to do the will of my Father, because my Father sent me.
14 And my Father sent me that I might be lifted up upon the cross; and after that I had been lifted up upon the cross, that I might draw all men unto me, that as I have been lifted up by men even so should men be lifted up by the Father, to stand before me, to be judged of their works, whether they be good or whether they be evil—
15 And for this cause have I been lifted up; therefore, according to the power of the Father I will draw all men unto me, that they may be judged according to their works.
16 And it shall come to pass, that whoso repenteth and is baptized in my name shall be filled; and if he endureth to the end, behold, him will I hold guiltless before my Father at that day when I shall stand to judge the world.
17 And he that endureth not unto the end, the same is he that is also hewn down and cast into the fire, from whence they can no more return, because of the justice of the Father.
18 And this is the word which he hath given unto the children of men. And for this cause he fulfilleth the words which he hath given, and he lieth not, but fulfilleth all his words.
19 And no unclean thing can enter into his kingdom; therefore nothing entereth into his rest save it be those who have washed their garments in my blood, because of their faith, and the repentance of all their sins, and their faithfulness unto the end.
20 Now this is the commandment: Repent, all ye ends of the earth, and come unto me and be baptized in my name, that ye may be sanctified by the reception of the Holy Ghost, that ye may stand spotless before me at the last day.
21 Verily, verily, I say unto you, this is my gospel; and ye know the things that ye must do in my church; for the works which ye have seen me do that shall ye also do; for that which ye have seen me do even that shall ye do;"

Based on the scriptures cited above, the fulness of the gospel, as delivered by the Savior, is;

The Lord summarizes this treatise of the fulness of the gospel in verse 20: Repent, come unto me, be baptized, receive sanctification through the Holy Ghost. The theme of repent and come unto me is found throughout the scriptures. The message here is that we must repent of our sins and then seek Him out. I will touch on this later