The occasion for this letter is Moroni’s call “to the ministry.” The fact that it was commemorated with a congratulatory letter confirms that it was a specific event that marked Moroni’s transition from not being “in the ministry” to being “called to the ministry.” It must have been something like the modern calling and setting apart—or perhaps ordination.
The fact of the letter tells us that Mormon considered this calling to be an honor, something extraordinary. But the content is doctrinal, not personal, suggesting that Mormon sees Moroni as being in a position to do something about that doctrine. The Book of Mormon has mentioned only three Nephite priesthood offices: teacher, priest, and elder (or apostle). While teachers may teach correct doctrine, they do not have authority over other teachers (at least, as we understand their office), suggesting that Moroni may have more authority than a teacher. Because he has received visitations from angels, the Three Nephites, and the Lord, I hypothesize Moroni has been called as an elder (apostle). That certainly puts him in the position to have authority over the doctrines taught in the church. A priest might have similar authority but it would probably have been more localized. As an elder (apostle), Moroni would have a wider range of authority and, therefore, opportunity to check heresies.