In his history, Joseph reported that the angel told him about a book written on gold plates. "He also said that the fulness of the everlasting Gospel was contained in it, as delivered by the Savior to the ancient inhabitants" (JS-H 1:34). So if we believe the angel, the gold plates contain the fulness of the gospel. This is repeated several times in the D&C.
In the first paragraph of the introduction to the Book of Mormon, the editors extended this statement, saying, "the Book of Mormon...contains the fulness of the everlasting gospel."
Is that true?
If the fulness of the gospel is one of the things in the book written on gold plates (often referred to in the revelations as the Book of Mormon), but today's Book of Mormon is only an extract from the original, what guarantee do we have that we have that the book published by the church today actually, truly contains the fulness of the everlasting gospel?
I realize that in the D&C, the Lord says multiple times that the "Book of Mormon" contains the fulness. But after rereading those passages, I'm not convinced that he is referring to what Joseph translated and published--I think it more likely that he means the entire book Mormon wrote.
In general, I think we are too quick to congratulate ourselves for all the truth we have, when the scriptures are clear that (1) there is a ton more that needs to be revealed, and (2) we are doing a pretty terrible job with what has already been revealed.