“For in that day” explains why the “kings shall shut their mouths” in amazement. As prophesied, they will see “a great and a marvelous work among them.” This language echoes Isaiah 29:14: “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 that “great and a marvelous work.” Nephi1 had developed this same theme, using the same “marvelous work and a wonder” imagery (2 Ne. 25:17).
However, despite the prophecy (“although a man shall declare it unto them”), some would “not believe it.” Although “a man” specifically refers to Joseph Smith, this passage also applies to the missionaries, who spread the news of this marvelous work and a wonder to a skeptical world.