This letter comes on an occasion. That occasion is noted by Moroni in the first verse, and then specifically stated in the beginning of the letter. The occasion is the “call to the ministry.” We do not have the information to give the precise definition of what that call would be, but the fact that it is noted as an occasion and specifically mentioned by Moroni in the introduction leads us to some speculations.
First, the fact that it is noted as a particular time, and marked by a congratulatory letter, tells us that this is a specific event. Moroni was not “in the ministry,” and then is “called to the ministry.” This must correlate to the modern calling and setting apart – or perhaps ordination. In any case, Moroni first was not, and now is, called to the ministry.
Second, the fact that Mormon would write a congratulatory letter tells us that Mormon considered this an honor, and something not particularly ordinary. For instance, in modern LDS practice, the tone of the letter does not suggest the ordination of a deacon to the Aaronic priesthood, an event that while important, is still reasonably common. Of course even that occasion is worth special note, but this particular letter has the feel of something more out of the ordinary.
Lastly, the content of the letter is doctrinal, suggesting that Mormon now believes that Moroni is in a position to do something about that doctrine. In our known catalog of Nephite priesthood positions, we have only three; teacher, priest, and elder (with elder being the equivalent of Apostle). The fact that Mormon feels that Moroni can do something about controlling false doctrine suggests more than a teacher. While teachers may teach correct doctrine, they do not have authority over other teachers of doctrine. That leaves priest and elder. With our understanding of Moroni’s visitations from angels and the Lord, it would seem that Moroni has been called as a elder. That certainly puts him in the position to have authority over the doctrines officially taught in the church. While a priest might have had similar authority, it would have been more localized. As an elder, Moroni would have a wider range of authority, and therefore opportunity to eliminate a rising heretical doctrine.