As the official recordkeeper, Alma gives Helaman a new command. He is to write a prophesy, but to keep that prophecy from the people until it is fulfilled. One of the reasons is that the prophecy is quite time specific. Four hundred years from the time that the Messiah will show himself to the Nephites, the Nephite nation will dwindle in unbelief. Although Alma will be explicit that this will lead to their destruction, Helaman might have already surmised that much, because the promise of the land depended upon continued righteousness. Dwindling in unbelief surely would lead to some kind of destruction.
We cannot know how this prophecy was kept from general knowledge, but if it was to wait to be known until its fulfillment, then Mormon would be the first to publish it widely. It would not have been earlier, since it was Mormon who lived in the time of its fulfillment.
If the Book of Mormon is set in a Mesoamerican location, the very fact that this destruction comes four hundred years after the manifestation of Christ is significant. Rather than simply be a prophecy of a particular time, it was a very significant time. The Mesoamericans used counting system based on 20 rather than 10. For them, a four-hundred-year period carried much the same kind of cognitive significance as a thousand years in our decimal system. The Maya named that period a baktun, just as we name our significant period a millennium.