The theme of present/future contrast continues. There is no subject continuity. There is not supposition that there is any relationship between the poor and those who mourn. What there is, is the idea that there are many kinds of people, all of whom will see a reversal from their condition in this life to the next.
The contrast here is between the mourning and the comforting. There are two aspects of this contrast that important. The first is that there is a contrast between the feelings of those who mourn, and the feelings of those whose mourning will become comforted. The direct parallel would be to the feeling, some future amelioration of the pain that is felt after a death. Certainly this particular beatitude was particularly appropriate in the Mesoamerican context at this particular time. Even with a year having passed, there would still be the pain of the lost loved ones, and the promise of comfort would be real.
However, there is a more important part of this promise. It is too simplistic to read this beatitude as the simple contrast of feelings. The very concept of the contrast is that there is a tremendous difference between the before and after conditions. In the case of the poor, those who had little were to inherit everything. That would appear to make the promise “you will feel better” an insignificant blessing in comparison.
This is not an insignificant blessing, however, for it relies upon a condition of heaven that is not present, and that is integral to the message of the Atoning Messiah. The connection between the comforting and the Messiah was so strong in Israel that rabbinic tradition used the title of “Comforter,” as one of the descriptors of the Messiah. (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 81) Part of what the Book of Mormon prophets have preached concerning this event was the resurrection from the dead, and it is that reality that provides the true nature of the blessing to those who mourn. They mourn because a loved one is dead. They will be truly comforted, not just made happy, because that loved one will yet live. The very cause for temporal mourning will be removed.
Textual: The 3 Nephi text follows the Matthean set of the Beatitudes. This particular Beatitude has a parallel in Luke that has a similar concept, but slightly different connotations. In Luke we have:
Luke 6:21
21 … Blessed are ye that weep now: for ye shall laugh.
The weeping parallels the forms of mourning. The laughing is a direct emotional antithesis of the weeping, strengthening the assumption that the comfort is also in some way an antithesis to the mourning in Matthew.