Mormon informs us that he had been going to include in his record all of the marvelous teachings the Savior gave to the multitude that are recorded in the larger library of Nephite records known as the large plates of Nephi, but that he was forbidden of the Lord. “I will try the faith of my people.” the Lord said.
This is not some cruel game that God lays with mankind. It is, rather, an act of love and mercy for us. For our own sake—our spiritual development and salvation — the Savior expects us to develop faith and righteousness by trusting in and following the teachings of the portions of his word that he has already revealed—as in our standard works and in the words of prophets and Apostles of our dispensation. He desires us to study, ponder, pray about, and heed those teachings we have so that we will desire more and be spiritually prepared to have “greater things” manifest to us.