The phrase “cried unto them from the dust” is an image of the record of a people who have died, but were still able to speak. It is a more modern phrasing that is perhaps based on Isaiah 29:4 “And thou shalt be brought down, and shalt speak out of the ground, and thy speech shall be low out of the dust, and thy voice shall be, as of one that hath a familiar spirit, out of the ground, and thy speech shall whisper out of the dust.” The Book of Mormon usage pulls the basic phrase and meaning of the dust as equivalent to the grave. When “it shall be as if the fruit of thy loins had cried unto them from the dust,” the image is that the coming forth of the Book of Mormon will be as though the Nephites themselves were speaking. The testimony of that people will speak to a modern time, even though the Nephites themselves had passed to the dust.
The idea that the things of God are simple things that will become strong is repeated several times in the Book of Mormon, and the intended meaning is that human means will provide divine instruction. Humankind may be simple and weak, but God is strong and may use humankind to further his eternal purposes.