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Question: Does the Book of Mormon support trinitarianism?
Question: Does the Book of Mormon support trinitarianism?
A case can be made against this with a couple of passages
The Book of Mormon begins (1 Nephi 1:8-10) with Lehi's vision of God on his throne. One [Christ] followed by twelve others descends from God to speak with Lehi--thus, Jesus and the Father are here both separate, and the role of Christ in giving instructions to the prophet while the Father looks on and approves is followed, just as it was in Joseph's First Vision. Here too, Lehi is described as praying to "the Lord," and yet has a vision of both God the Father and Christ.
Alma 11:45 makes clear that the resurrection is permanent and Mosiah 15:20 (along with several others) makes clear that the resurrection is brought about through Christ.
In 3 Nephi 28:10 the Savior is speaking to the 3 Nephites. After declaring that they would never endure the pains of death he states:
“10 And for this cause ye shall have fullness of joy; and he shall sit down in the kingdom of my Father; yea, your joy shall be full, even as the Father hath given me fullness of joy; and ye shall be even as I am, and I am even as the Father; and the Father and I are one;”
Since the verse is juxtaposed closely with not tasting death and the Savior stating that they would be even as he and the Father are, this verse may be used to argue for an embodied Christ and God (and likely an early conceptualization of deification) in the Book of Mormon. Furthermore, the phrase “fullness of joy” is used in D&C 93:33 (dated to 1833) to describe element (or man’s tabernacle as v. 35 expresses) and spirit inseparably connected.
Critics of Mormonism have given little attention to the theology represented by the original text, merely reading into the text modern ideas about trinitarianism
As for other points of the text, some critics assert that the original Book of Mormon text supported trinitarianism, and that Joseph Smith's edits to the book were attempts to change this. While critics tend to focus on changes made by Joseph Smith to the Book of Mormon, they have given little attention to the theology represented by the original text, merely reading into the text modern ideas about trinitarianism. Brant Gardner has pointed out, “The problem is that when we focus on the fact of the change, we automatically ask the wrong questions.”  Gardner goes on to explain:
our first obligation [is] to understand what this passage tells us about Nephite theology. Before we worry about how to explain the Book of Mormon to make it fit our current descriptions of God, we really should understand how the Nephites described and understood God. To do this, we must approach the question not only critically, but historically. 
In the original text, Nephi speaks of a Messiah who is “God,” (1 Nephi 11:18), “the Eternal Father,” (1 Nephi 11:21), “the everlasting God,” yet is “the Son of the most high God” (1 Nephi 11:6). This Messiah-God/Son of God (Most High) becomes a man, suffers, and even dies “for the sins of the world” (1 Nephi 11:33). Later Nephi would summarize, “the God of our fathers, who were led out of Egypt, out of bondage, and also were preserved in the wilderness by him, yea, the God of Abraham, and of Isaac, and the God of Jacob, yieldeth himself, according to the words of the angel, as a man, into the hands of wicked men” (1 Nephi 19:10). In approaching these concepts critically and historically, as Gardner recommends, we need to understand how they might have been understood by an Israelite around 600 BC.
As scholars have explored the origins of Christianity, it is becoming increasingly apparent that it is deeply rooted in the earliest forms of Israelite religion
According to Daniel Boyarin, a leading scholar of Judaism, that “in the very moments that we take to be most characteristically Christian as opposed to Jewish,” we actually find some of the earliest conceptions of Israelite religion. These include:
the notion of a dual godhead with a Father and a Son, the notion of a Redeemer who himself will be both God and man, and the notion that this Redeemer would suffer and die as part of the salvational process. At least some of these ideas, the Father/Son godhead and the suffering savior, have deep roots in the Hebrew Bible as well and may be among some of the most ancient ideas about God and the world the Israelite people ever held. 
These are, notably, the very ideas portrayed to Nephi in vision.
Margaret Barker has attempted to reconstruct the religion of the Israelites during the time-period before the Babylonian exile. According to her, Yahweh (Jehovah) was both the God of Israel, and also the son of the Most High God (El Elyon), and manifest on earth in human form as the Messiah.  Brant Gardner summarizes her reconstruction:
- A Father-God, ’El who is also called el elyon or “Most High God.”
- A heavenly council of the sons of God.
- Yahweh as the son of God (El).
- Yahweh as preeminent God of Israel.
- Yahweh as Messiah. 
According to Barker, “The original temple tradition was that Yahweh, the Lord, was the Son of God Most High, and present on earth as the Messiah.”  Once again, these are the very concepts expressed in Nephi’s vision.
While being the son of El Elyon, Yahweh was the father of Israel through covenant.  This same kind of relationship is expressed in the Book of Mormon (e.g., Mosiah 3:7). As Brant Gardner explains, “Yahweh becomes the father as he acts in the vertical deity-to-mortal realm … the people’s covenant create a new relationship with God.” 
The explanation for this conflation of Father and Son cannot be found in post-Christian theologies of modalism or trinitarianism
The theological setting indicated by Boyarin, Barker, and Gardner provides a conceptual framework that explains how and why Nephi would refer to the Messiah both as God and as the son of the Most High God, and even as father. Gardner thus concludes:
The explanation for this conflation of Father and Son cannot be found in post-Christian theologies of modalism or trinitarianism. However, by reading these passages against the Nephite [i.e., ancient Israelite] cultural context, we can understand why Nephi could hold what appear, to modern readers, to be contradictory beliefs about God. 
- Brant A. Gardner, Second Witness: Analytical and Contextual Commentary on the Book of Mormon, 6 vols. (Salt Lake City, Utah: Greg Kofford Books, 2007–2008), 1:214, emphasis in original.
- Gardner, Second Witness, 1:214.
- Daniel Boyarin, The Jewish Gospels: The Story of the Jewish Christ (New York: The New Press, 2012), 158.
- See Margaret Barker, The Great Angel: A Study of Israel’s Second God (Louisville, Kentucky: Westminister/John Knox Press, 1992).
- Gardner, Second Witness, 1:215.
- Margaret Barker, “Joseph Smith and Preexilic Israelite Religion,” in The Worlds of Joseph Smith: A Bicentennial Conference at the Library of Congress, ed. John W. Welch (Provo, Utah: BYU Press, 2006), 79.
- Barker, The Great Angel, 4–10; Gardner, Second Witness, 1: 217-222.
- Gardner, Second Witness, 1:221.
- Gardner, Second Witness, 1:218.