Mormon transitions from speaking of the elements to speaking of humankind. He does this by speaking of an action that humans would do, but where the elements might respond to God and override the human’s action. He is specifically referring to the burying of treasure in the earth. A person may bury treasure, but if God says that it is accursed, it is accursed, and it might never be found.
This allows Mormon to shift from the elements, which are faithful, to the humans who are not. If the treasure was accursed, and not to be found, if a person is unfaithful, “because of [his] iniquities, [he] shalt be accursed forever.”
In spite of this gloomy message, Mormon does not leave us without hope. It is precisely because of these consequences to our unfaithfulness that “repentance [has] been declared.”