According to McConkie and Millet, reference to "priests and teachers" in the Book of Mormon should not be confused with the office of priest or the office of teacher as known to us in the Aaronic Priesthood today. It is believed that the Aaronic or Levitical Priesthood did not exist among the Nephites unless it was brought during Christ's visit among them. [Joseph Fielding McConkie and Robert L. Millet, Doctrinal Commentary on the Book of Mormon, Vol. 1, p. 225]
“Nephi Did Consecrate Jacob and Joseph Priests and Teachers”
In 2 Nephi 5:26 we find that "Nephi did consecrate Jacob and Joseph . . . priests and teachers." According to Reynolds and Sjodahl, under the Mosaic law, the priests were appointed to offer up sacrifices for the people as well as for themselves. (Leviticus 4:5,6) Under the law, priests were also teachers of the people, as appears from the following:
Do not drink wine or strong drink, thou, nor thy sons with thee, when ye go into the tabernacle of the congregation, lest ye die; it shall be a statute for ever throughout your generations: and that ye may put difference between holy and unholy, and between clean and unclean; and that ye may teach the children of Israel all the statutes which the Lord hath spoken to them by the hand of Moses. (Leviticus 10:9-11)
[George Reynolds and Janne M. Sjodahl, Commentary on the Book of Mormon, Vol. 1, p. 279]
“Priests”
The Book of Mormon states that Lehi and his family were descendants of Joseph who had been sold into Egypt (2 Nephi 3:4). Yet, the children of Lehi were ordained priests (2 Nephi 5:26).
Richardson, Richardson and Bentley write that when the faultfinders discovered these verses, they immediately referred to Bible passages that designate the Levites as those who were to perform priestly duties. (For example see Deuteronomy 21:5) In fact the Mosaic Law specified that death was the penalty for any "stranger" who would attempt to perform priestly duties (Numbers 3:10). More specifically, only Aaron and his sons, who were descendants of Levi, could hold the office of priest. Their duties included offering sacrifices, burning incense, teaching the law, transporting the Ark of the Covenant, etc.
However, the Bible tells of other men, not of the tribe of Levi, who offered sacrifices in place of a priest. Gideon, who was not a Levite but an Abi-ezrite, lived in the territory of Manasseh. He was not a priest but a warrior--"a mighty man of valour" (Judges 6:11-15). Yet, he was commanded by the Lord to offer a sacrifice upon an altar (Judges 6:26) just as a Levitical priest would do. An angel of God instructed Manoah, who was of the tribe of Dan to sacrifice burnt offerings to the Lord (Judges 13:2, 15:21). David, another non-Levite (of the tribe of Judah) offered sacrifices and blessed the people (2 Samuel 6:18; 1 Chronicles 16:2-3) as if he were a Levitical priest. (See also Mark 2:25-28, where Jesus recognized and accepted David's priestly acts.) The Bible also mentions that king David's sons, were priests (2 Samuel 8:17). In fact, the Bible specifically differentiates between priests and Levites (1 Kings 8:4; Ezra 2:70; John 1:19). Isaiah records the word of the Lord saying that He would make gentiles priests and Levites (Isaiah 66:19-21).
Therefore, those who criticize the Book of Mormon for allowing non-Levites to perform priestly functions are indirectly criticizing their own Bible for doing the same. [Allen H. Richardson, David E. Richardson and Anthony E. Bentley, 1000 Evidences for the Church of Jesus Christ of Latter-day Saints: Part Two-A Voice from the Dust: 500 Evidences in Support of the Book of Mormon, pp. 129-130]
“I Nephi Did Consecrate Jacob and Joseph That They Should Be Priests and Teachers”
According to Daniel Ludlow, many references in the Book of Mormon indicate that the Nephites held priesthood--that is, they had the power and authority to act in the name of God. However, the Book of Mormon does not refer specifically to the two major divisions in the priesthood, the "Aaronic Priesthood" and the "Melchizedek Priesthood." Thus the question has frequently arisen as to exactly what priesthood was held by the Nephites. Joseph Fielding Smith gives his answer in the following comprehensive statement:
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. (Alma 10:3; The Improvement Era, Vol. 8, p. 781; Journal of Discourses, Vol. 23, p. 184.) 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. It is true that Nephi "consecrated Jacob and Joseph, that they should be priests and teachers over the land" of the Nephites (2 Nephi 5:26), but the fact that plural terms "priests" and "teachers" were used indicates that this was not a reference to the definite office in the priesthood in either case, but it was a general assignment to teach, direct, and admonish the people. Otherwise, the terms priest and teacher would have been given, in the singular. Additional light is thrown on this appointment showing that these two brothers of Nephi held the Melchizedek Priesthood, in the sixth chapter, second verse of 2 Nephi, where Jacob makes this explanation regarding the priesthood which he and Joseph held: "Behold, my beloved brethren, I, Jacob having been called of God, and ordained after the manner of his holy order, and having been consecrated by my brother Nephi, unto whom ye look as a king or a protector, and on whom ye depend for safety, behold ye know that I have spoken unto you exceeding many things" (2 Nephi 6:2). . . . (Joseph Fielding Smith, Answers to Gospel Questions, Vol. 1, pp. 124-126)
This seems to be a confirmation of the ordinations that Jacob and his brother Joseph received in the Melchizedek Priesthood. [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, pp. 132-133]
“I Nephi Did Consecrate Priests and Teachers”
According to John Welch, an essential part of the temple ascension of new potentates in the ancient world was to install temple priests and administrators who would rule under the new king. We see that Nephi consecrated Jacob and Joseph to "be priests and teachers" (2 Nephi 5:26). This consecration repeated later in the Book of Mormon when King Mosiah2 became king and when priests were appointed as the first official act of the new coregency (see Mosiah 6:3).
Of course, these Nephite priests were not priests or Levites by birth. They were ordained "after the manner of [God's] holy order" (2 Nephi 6:2). The persistence of that phrase in the Nephite record (Alma 6:8; 13:1,8,10-11) shows that the Nephites consciously based their priesthood authority on principles lodged in God's holy order, rather than in tribal rights or inheritances. Indeed, they looked to Melchizedek as the paragon of priesthood (see Alma 13:14-19), probably in large part because Melchizedek was the most conspicuous priest in the Pentateuch who was not a Levite.
But Melchizedek lived before the time of Moses, and so one might well wonder how Lehi could rightly purport to live the law of Moses without having Levites to officiate in the sanctuary. If Lehi or Nephi ever struggled with this issue, they gave no indication to that effect; and we can easily imagine several reasons why they did not:
1. Revelation guided Nephi. . . .
2. The Nephites may have viewed the priestly inheritance of the Levites as belonging only to the temple in Jerusalem; . . .
3. By returning to the typology of the exodus from Egypt, Lehi's colony assumed a posture that had previously recognized all of Israel as "a kingdom of priests, and an holy nation" (Exodus 19:6). . . .
4. Although the history of the priesthood in ancient Israel is complicated and obscure, it is clear that certain priests, such as Zadokites and Gibeonites, officiated in the temple of Solomon in addition to Levites. Aelred Cody notes that "if Ezekiel 44:6-10 condemns the practice of having uncircumcised foreigners serving in the temple, it is because the practice existed."
5. Nephi may simply have viewed the appointment of priests as a rightful prerogative of the king. (King David appointed priests, including his sons--see 2 Samuel 8:15-18; 20:25-26). . . .
6. The term "Levite" may well have been a functional title in addition to a genealogical one. . . . Bright explains: "'Levite' was also a functional designation meaning "one pledged by vow'; men of any clan thus dedicated to Yahweh could become Levites. In the course of time, many priestly families and individuals not of Levitic lineage were so reckoned because of their function--as was Samuel (1 Chronicles 6:28)."
[John W. Welch, "The Temple in the Book of Mormon," in Temples of the Ancient World, pp. 331-333]