The subject of Priesthood among the Nephites is an interesting one. At first glance, it would appear that Jacob and Joseph were ordained to be priests and teachers in the Aaronic Priesthood. However, there was no Aaronic Priesthood prior to Christ among the Nephites. During Mosaic times, only descendants of the tribe of Levi held the Levitical or Aaronic Priesthood. Since the Nephites and Ismaelites were descendants of Manasseh and Ephraim, respectively, they were not heirs to the Levitical Priesthood. Therefore, Nephi, Jacob and Joseph held the Melchizedek Priesthood. Joseph Fielding Smith said:
"The Nephites were descendants of Joseph. Lehi discovered this when reading the brass plates. He was a descendant of Manasseh, and Ishmael, who accompanied him with his family, was of the tribe of Ephraim. Therefore there were no Levites who accompanied Lehi to the Western Hemisphere. Under these conditions the Nephites officiated by virtue of the Melchizedek Priesthood from the days of Lehi to the days of the appearance of our Savior among them." (Answers to Gospel Questions, 5 vols. [Salt Lake City: Deseret Book Co., 1957-1966], 1: 124.)
While some have argued that the term "priests and teachers" referred not to offices of the priesthood but rather to callings in the church, the scriptural evidence in the Book of Mormon does not support this contention (see Jarom 1:11, Mosiah 23:17; 25:19, Alma 4:7; 6:1; 15:13; 23:4; 45:23, Moro 3:1). Can a priest be a priest without priesthood? Of course not. Among the Nephites, the offices of priest and teacher were ordained offices of the Melchizedek Priesthood. This is what Alma taught, 'I would that ye should remember that the Lord God ordained priests, after his holy order, which was after the order of his Son, to teach these things unto the people' (Alma 13:1, italics added, see also DC 76:57, and commentary for Moroni 3:1)
"Jacob reminds us that he and his brother Joseph had been appointed priests and teachers among the people under the hands of Nephi (see "#2 ne. 5:262 Nephi 5:26). These callings are descriptive of their labors in the Melchizedek Priesthood, rather than offices in the Aaronic Priesthood as we know them. So far as we know, there was no Aaronic Priesthood among the Nephites, until, possibly, the coming of Christ to America. (See Promised Messiah, p. 412; New Witness, P. 348.)" (Joseph Fielding McConkie and Robert L. Millet, Doctrinal Commentary on the Book of Mormon, 4 vols. [Salt Lake City: Bookcraft, 1987-1992], 2: 9.)