6 And now there cannot be written in this book even a hundredth part of the things which Jesus did truly teach unto the people;
7 But behold the plates of Nephi do contain the more part of the things which he taught the people.
8 And these things have I written, which are a lesser part of the things which he taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken.
9 And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.
10 And if it so be that they will not believe these things, then shall the greater things be withheld from them, unto their condemnation.
11 Behold, I was about to write them, all which were engraven upon the plates of Nephi, but the Lord forbade it, saying: I will try the faith of my people. (3 Nephi 26:6-11)
This is one of those bits that intrigues us. We wonder what greater things we might be given of the things Jesus ministered to the Nephites. We might think that we haven’t yet gotten the greater things.
Recently when reading this I just realized that the Doctrine & Covenants pretty well qualifies as “greater things,” in comparison to the words of Christ given in the Book of Mormon.
And it is as Jesus says – if people don’t believe the Book of Mormon and the words of Christ in it that were revealed through the prophet Joseph Smith, they certainly aren’t going to believe the Doctrine & Covenants, which is, the biggest part of it, the words of Christ, also revealed through Joseph Smith.
The D&C teaches foundational principles of the gospel but then also gives so many mysteries of God (degrees of glory, vicarious ordinance work, eternal marriage, priesthood organization, etc.) and expounds many things, so how different would that be from what the Lord would teach the Nephites? He gave them the Beatitudes like He gave the Jews, so why not also the equivalent of the contents of the D&C as well?
This is not to say that more beyond that can’t be given, but I think we can say with some surety that the Book of Mormon does test and try the people as to whether they have the faith to receive the D&C and more words of Christ. Furthermore, our faith also determines whether we act as though "whether by mine own voice or by the voice of my servants, it is the same" and receive the words of the church leaders in conferences.