[For in that day] This verse serves as the explanation of why the “kings shall shut their mouths; for that which hade been told them shall they see…”
[which shall be a great and a marvelous work among them] The language here echoes Isaiah 29:
Isaiah 29:14
14 Therefore, behold, I will proceed to do a marvellous work among this people, even a marvellous work and a wonder: for the wisdom of their wise men shall perish, and the understanding of their prudent men shall be hid.
The Book of Mormon is the “great and a marvelous work.”
[there shall be among them those who will not believe it, although a man shall declare it unto them.] The prophecy is that even though the coming forth of the Book of Mormon is a “great and a marvelous work,” it will not be universally believed. There will be disbelievers. Those who “declare it unto them,” are all of the missionaries who spread the news of this marvelous work and a wonder, although the specific reference is to Joseph Smith.