At first glance, I wondered if this phrase applied to the Book of Mormon and Bible individually, collectively, or just to the Book of Mormon. After doing a search for “fulness” and “gospel,” it seems obvious that the scriptures talk about the Book of Mormon having a “fulness” of the gospel, but that the Bible no longer does. (See this search.) Interestingly, many of these references also talk about the fulness of the gospel as a covenant. This would seem at first to be separate from the Book of Mormon references, except that in D&C 84 we are told to remember “the new covenant, even the Book of Mormon.”
There is also something intriguing in this verse in D&C 76:
“Of whom we bear record; and the record which we bear is the fulness of the gospel of Jesus Christ, who is the Son, whom we saw and with whom we conversed in the heavenly vision.”
Is the fulness of the gospel a clear testimony of who Christ is, and what the atonement is? Is the Bible not “full” in that it somehow lost this very thing?
The Book of Mormon over and over again testifies of the reality/divinity of Christ – Nephi, Jacob, Isaiah, Mormon, Moroni, Nephi (in Helaman), not to mention the whole book of 3 Nephi. And its several discourses on the atonement are very clear and bear strong witness. Is this why the Book of Mormon has a fulness of the gospel? What is the gospel? A clear call to Christ and baptism?
Doing a search for fulness and covenant makes me think there really is something to connection between the fulness of the gospel and a covenant.
It seems the fulness of the gospel is what is sent forth in order to restore Israel – and that of course is the covenant, that they will be restored. Or restoring them is a fulfillment of the covenant? But either way there is some sort of equivalence between the covenant and the fulness of the gospel:
D&C 66:2: “blessed are you for receiving mine everlasting covenant, even the fulness of my gospel”
D&C 39:11: “Thou shalt preach the fulness of my gospel, which I have sent forth in these last days, the covenant”
D&C 133:57: “the Lord sent forth the fulness of his gospel, his everlasting covenant”