"By their fruits ye shall know them."
The Book of Mormon, the restoration of sacred ordinances, the charity, everything that every prophet since has done to further the work. These are Joseph Smith's fruits. The peace and the light they bring are as powerful as the cosmos. How can anyone deny the truth of things so majestic? And if these things are true, then Joseph Smith must be a prophet, for nothing so immense could come from a man. They can only come from God, who has power over all of Creation.