Modalism (Oneness Theology) in the Book of Mormon:
As a matter of historical recoded, Leader and Founder, and first so-called Prophet and President of the Mormon Church Joseph Smith could not keep consistent in his theology. For example on the nature of God, he went from ‘one God’ (Book of Mormon [cf. Alma 11:44; 2 Nephi 31:21, Testimony of the (so-called) Three Witnesses, early sections of D&C, Book of Abraham], to Modalism (sections in the Book of Mormon [cf. Mosiah 15:1-5; Ether 3:14; Alma 11:38-39], to flat out polytheism (cf. Teachings of the Prophet Joseph Smith, pp. 345-347), which the LDS Church embraces today. See Early Teachings of Joseph Smith and the Book of Mormon, which Contradict Present- day LDS Theology
In terms of Modalism, Joseph Smith did not understand the difference between the doctrine of the Trinity and the teachings of Modalism. Modalism (also referred to as Oneness theology) was the second century heresy that asserted that God is unitarian (unipersonal), that is, God existing as one person that reveals himself in different modes, manifestations or dimensions, rejecting the Trinity.
In other words, in Oneness thinking, since God is one, and Jesus is called God, Jesus then is the Father, Son, and the Holy Spirit—not three persons, rather three manifestations or modes. Oneness doctrine teaches then that the unipersonal God (named Jesus) has two natures, divine being the Father and human being the Son. Thus, in this doctrine, Jesus acts sometimes as the Son (human) and sometimes as the Father (God) and yet other times the Holy Spirit.- – For more details on Oneness see: Oneness Theology.
So, what does Modalism have to do with Joseph Smith? Answer: the Book of Mormon teaches both Trinitarian and Modalism. However, I find that the Book of Mormon is more modalistic than Trinitarian, though. First, observe these decidedly modalistic passages in the Book of Mormon.
Mosiah 15. The introduction of Mosiah chapter 15 reads: How Christ is both the Father and the Son–He shall make intercession and bear the transgression of his people. . . .” Then, starting at verse 1 through verse 5, note the heighted areas:
And now Abinadi said unto them: I would that ye should understand that God himself [the Father] shall come down among the children of men, and shall redeem his people. And because he dwelleth in flesh he shall be called the Son of God, and having subjected the flesh to the will of the Father, being the Father and the Son. The Father because he was conceived by the power of God; and the Son, because of the flesh; thus becoming the Father and Son. And they are one God, yea, the very Eternal Father of heaven and earth. And thus the flesh becoming subject to the Spirit, or the Son to the Father, being one God. . . . (Mosiah 15:1-5).
That Jesus is the Father, is a teaching that is clearly taught in Smith’s, Book of Mormon
Ether 3:14: “Behold, I am the he who was prepared from the foundation of the world to redeem my people. Behold, I am the Father and the Son. . . .”
Alma 11:38-39: “Now Zeezrom saith again unto him: Is the Son of God the very Eternal Father? And Amulek said unto him: Yea, he is the very Eternal Father of heaven and of earth, and all things which in them are; he is the beginning and the end, the first and the last.”
Amazingly, a mere five verses later (11:44), we find a contradictive implication of what resembles the orthodox doctrine of the Trinity:
but every thing shall be restored to its perfect frame, as it is now, or in the body, and shall be brought and be arraigned before the bar of Christ the Son, and God the Father, and the Holy Spirit , which is one Eternal God. . . .”
2 Nephi 31:21: “And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end. Amen.”
Further, the “Testimony of the three Witnesses” do not agree with the present-day LDS teaching (three separate Gods). Rather, the converse is stated: “And honor be to the Father, and to the Son, and to the Holy Ghost, which is one God” (Book of Mormon, Introduction). The nature of God is not the only doctrine that Joseph Smith changed in his lifetime. However, the true God of biblical revelation is the triune God and thus, a denial of the nature of God is a denial of Christ and His gospel (cf. Isa. 43:10; Hosea 6:6; John 8:24, 58; 5:24; 17:3; 1 John 5:20).