The angel addressed Joseph and Oliver as “my fellow servants,” mirroring the introduction of the angel speaking to John in the book of Revelation (Revelation 22:9). In the service of the Lord, an angel of glory and a mortal person are seen as equals. One of the later teachings of Joseph Smith was that angels are human beings in a different phase of their eternal progression (see D&C 129), which is here highlighted by the fact that the angel explained that in mortality he was known as John the Baptist. The angel gave the keys associated with the Aaronic Priesthood, each of which deserves careful consideration:

The keys of the ministering of angels. The Hebrew and Greek words translated as “angel” in the Old and New Testaments could also be translated as “messenger” and can refer to both human and heavenly messengers (“Angel,” Lexham Bible Dictionary). Angels of glory can be premortal spirits, the spirits of just men made perfect, or resurrected beings of flesh and bone (D&C 129:1–3). As a premortal spirit, Jesus Christ served as a messenger on several occasions, including His ministry to the brother of Jared (see Ether 3), to Jacob (Genesis 32:30), to Moses (Exodus 33:11), and to numerous other prophets before He came in the flesh. When holders of the Aaronic Priesthood are asked by those in authority to serve as ministers, they are effectively serving as ministering angels.

The keys of the gospel of repentance and baptism by immersion for the remission of sins. President Dallin H. Oaks taught,

What does it mean that the Aaronic Priesthood holds “the key of the ministering of angels” and of the “gospel of repentance and of baptism, and the remission of sins”? The meaning is found in the ordinance of baptism and in the sacrament. Baptism is for the remission of sins, and the sacrament is a renewal of the covenants and blessings of baptism. Both should be preceded by repentance. When we keep the covenants made in these ordinances, we are promised that we will always have His Spirit to be with us. The ministering of angels is one of the manifestations of that Spirit. (“The Aaronic Priesthood and the Sacrament,” October 1998 General Conference).

This shall never be taken again from the earth, until the sons of Levi do offer again an offering unto the Lord in righteousness. In an 1840 discourse, while referring to the sacrifices offered anciently by Levitical priests, Joseph Smith taught, “These sacrifices as well as every ordinance belonging to the priesthood will, when the temple of the Lord shall be built, ​and the sons [of] Levi be purified​, be fully restored and attended to [in] all their power ramifications, and blessings—this ever did and will exist when the powers of the Melchizedek Priesthood are sufficiently manifest. Else how can the restitution of all things spoken of by all the Holy prophets be brought to pass?” (Instruction on Priesthood, circa 5 October 1840, JSP).

Joseph Smith’s teaching does not refer to a permanent return to the system of sacrifice utilized under the law of Moses. President Joseph Fielding Smith taught, “Provision will be made for some ceremonies and ordinances which may be performed by the Aaronic Priesthood and a place provided where the sons of Levi may offer their offering in righteousness.” He also suggested that “the law of sacrifice will have to be restored, or all things which were decreed by the Lord would not be restored.” If these offerings consist of blood sacrifices as the Levites practiced anciently, it will be only temporary. President Smith believed that “blood sacrifices will be performed long enough to complete the fulness of the restoration in this dispensation. Afterwards the sacrifice will be of some other character” (Doctrines of Salvation, 1956, 3:93–94). The sacrifices made after the restoration of all things will likely be of the nature described in the Book of Mormon where the Lord asked His people to “offer for a sacrifice unto me a broken heart and a contrite spirit” (3 Nephi 9:20).

The statement that this authority will never be taken until the sons of Levi give their offering should not imply that the priesthood will be taken after that time. In Oliver Cowdery’s remembrance of this event, he wrote that the angel said “this authority, which shall remain upon the earth, that the sons of Levi may yet offer an offering unto the Lord in righteousness” (“Letter I,” Latter Day Saints’ Messenger and Advocate, October 1834, 1:13–16). This reading seems to imply that rather than the return of the sons of Levi taking away the priesthood, the restoration of the priesthood was intended to facilitate the return of the sons of Levi and their renewed offering. In an interview given near the end of his life, Oliver reinforced this point, explaining that the angel “said at the same time that it [the Aaronic priesthood] should remain on the earth while the earth stands” (Richard Lloyd Anderson, “Reuben Miller, Recorder of Oliver Cowdery’s Reaffirmations, BYU Studies, 8:3 [1968], 278).

Later in his life, Joseph Smith connected Malachi’s prophecy about the sons of Levi making an offering unto Lord to proxy ordinances performed on behalf of the dead. Joseph first received a revelation instructing him to build a temple in the city of Zion (Doctrine and Covenants 84:31). Later Joseph quoted the prophecy (Malachi 3:2-3, 3 Nephi 24:2-3) and admonished the Saints, “as a church and a people, and as Latter-day Saints, [to] offer unto the Lord an offering in righteousness; and let us present in his holy temple, when it is finished, a book containing the records of our dead, which shall be worthy of all acceptation” (Doctrine and Covenants 128:24).