By “these things [that] should come unto their brethren,” Moroni means the Book of Mormon. By faith, Moroni’s Nephite forefathers obtained the promise that the gospel would be preached to “their brethren,” the Lamanites. One of the great purposes of the Book of Mormon—indeed, Mormon’s parting thought (Morm. 7)—is the redemption of the Lamanites. Yahweh granted this promise thanks to the faith of past prophets.