A Marvelous Book

The phrase "a marvelous work is about to come forth among the children of men" appears in Joseph's revelations starting in section four, and appears frequently in many of his other revelations. This phrase refers to the translation and publication of the Book of Mormon from the golden plates. Mormon belief states that Joseph was given engraved gold plates by an angel of God. These plates contained a history of the ancient people of America, but more importantly, it contained the teachings of the prophets among them as they taught the Gospel of Jesus Christ. Joseph was commanded to publish these records as the Book of Mormon. The Lord called it "a marvelous work"
near the beginning of Joseph's translation work.
While Joseph may have partially understood the importance that the Book of Mormon would play in the world, certainly he did not understand the scope of it all. He understood that the plates would be an instrument in the hands of the Lord, but did he know how much? I don't believe so. Joseph was led by his revelations as much as the other Saints were led by the same revelations. Receiving golden plates and being told to translate them must have seemed like a big enough task in itself. He hadn't had experience translating anything, and he was asked to translate what turned out to be 600 pages of religious and secular history (not including the 116 lost pages). I can only imagine his anxiety as the Lord called it a marvelous work--much bigger than what Joseph had imagined it to be up to this point. He did not know where the translation of this book would lead him, and how it would define and prove his prophetic mission.
So what was so "marvelous" about the translation? While Joseph translated a large number of pages in very little time, this was not the only marvel. Thousands of people in the United States and in Europe read the Book of Mormon and accepted it, without ever meeting the prophet that translated it. Even after Joseph's death, thousands of people flocked to the Rocky Mountains because of the spiritual power the Book of Mormon lent them. Even today, millions more are joining the church after reading parts or all of the Book of Mormon, without ever meeting the long-dead Joseph Smith. I saw many families converted in the mission field after reading from the Book of Mormon and praying to know of its truth. The Book stands as a testament of the truth that God has called a prophet to the earth once again, and that His Church has also been restored to the face of the earth.
In short, the marvelous work that the Lord spoke about is still happening, and the Book of Mormon is the engine and force behind that work. While Joseph Smith knew the Book of Mormon was true before he even translated it, I don't think he understood right away how important it would be for the Church or for the Lord's people. The Book of Mormon has stood the test of time, and it is truly a marvelous work, as the Lord stated nearly two centuries ago.