Textual: The prophecy is introduced with the phrase “and these are the words.” There are two possible meanings for that phrase. The first is that Alma speaks it to introduce the Lord’s words in the prophecy. The second is that they are Mormon’s words used to introduce Alma’s words from the plates. The second is the better interpretation. Not only does prophecy come to the prophet in his own words, but we have the interesting first phrase of the prophecy which says” “Behold, I perceive….” This phrase is perfectly at home in Alma’s description of his prophetic event, but hardly a term we expect from the Lord, whose statement would have been much more forceful than a perception.
The prophecy is that the Atoning Messiah will come, but that the people will fall away anyway four hundred years later. This prophecy is to be kept from the people because it prophesizes failure. With an understanding of future failure, it might deter the intermediate populations from striving as hard during their own lifetimes. Hope is a much better motivator than fatalism.
Cultural: It is possible that Alma and Helaman understood something sacred in the four hundred year period. The number four is a sacred number of completion in the Mesoamerican world, and therefore four-hundred years would be a “completion” number. Indeed, it was the “completion” of the Nephite people. This would have placed a stamp of finality on the prophecy of the ultimate destruction of the Nephite people.
One can only imagine how Mormon must have felt when he read and copied this fatalistic prophecy, as he was living the fulfillment of it. Mormon would have known that his efforts were in vain, and that he was foretold to fail. What courage it would have taken for him to carry on his task with the knowledge of eventual failure so surely before him.