Even though we have powerful words from Moroni, he nevertheless wrote often in insecurity. He worried that men would mock his words. He makes certain that any mistakes be accounted to men and not to God. It is under that concern that Moroni writes in verses 27–29.
Perhaps there will be some who will not believe what he has written. Therefore, he provides the final proof of the truth of what he has written. There will be a time when both Moroni and future readers will stand before God. At that time, it will be impossible to deny, for God himself “shall show unto you, that that which I have written is true.”