Moroni is presumably quoting here from words spoken by our Lord to the Nephites during his American ministry. They are not to be found in our present text, Mormon’s abridgment of the plates of Nephi, though we do have a record of the Master speaking such words on the Eastern Hemisphere (see Mark 16:15-18).
Mormon himself explained: “And now there cannot be written in this book even a hundredth part of the things which Jesus did truly teach unto the people; but behold the [unabridged, large] plates of Nephi do contain the more part of the things which he taught the people” (3 Nephi 26:6-7). This is an important part of Moroni’s argument-the words of the Lord himself decree in unmistakable language that signs and wonders always follow the faithful believers.