6 And now, if I have no authority for these things, judge ye; for ye shall know that I have authority when ye shall see me, and we shall stand before God at the last day. Amen.
The authors of the Book of Mormon want the reader to know that they are certain of the truthfulness of their words. Not only that, they want the reader to know that they will know it to be true as well. There are two ways this knowledge can come to the reader.
The first is through a humble prayer of the heart. Moroni has not written his famous promise, but he has already promised certain knowledge to the sincere listener.
The other path is through arrogance of heart and an unwillingness to listen. Those individuals will be brought to know of the truthfulness of the Book when they encounter Moroni at the judgment bar of God.
Moroni begs his reader to be in the former rather than the later category.