Answer
Mormons say they believe in the Father, the Son, and the Holy Spirit. However, Mormon doctrine denies the Trinity, teaching that the Father, Son, and Spirit do not comprise one God.
According to Mormonism, Jesus is a created being, the first spirit to be born of the Father (Mormon Doctrine, p.129) and a celestial mother (Mormon Doctrine, p.516). Therefore, Jesus could not be the eternal God or part of an eternal Trinity. Mormons also teach that both the Father and the Son are men with bodies of flesh and bone (Doctrine & Covenants 130:22; Articles of Faith, p 38); as two separate people, the Father and the Son cannot be considered “one.”
Mormons also teach that Jesus is just one of many sons of God. Jesus is referred to specifically as “a son of God” in the Book of Mormon (Alma 36:17). Lucifer, or the devil, is another son of God in Mormon theology (Mormon Doctrine, p.163). Further, Mormonism teaches that the number of gods is increasing. Any man on Earth can one day become the god of another planet and populate it with children born to him from his eternal wife (Teachings of the Prophet Joseph Smith, 345-354). Any one of those children can later become a god in his own right (Doctrine & Covenants 132:20). Thus, there is not just One God, triune or not; there are many, many gods (Book of Abraham 4:3).
Mormonism’s founder, Joseph Smith, clearly rejected the Trinity. He wrote, “Many men say there is one God; the Father, the Son and the Holy Ghost are only one God. I say that is a strange God. . . . All are crammed into one God according to sectarianism [the Christian faith]. It would make the biggest God in all the world. He would be a wonderfully big God—he would be a giant or a monster” (Teachings, p. 372). Other Mormon writers such as James Talmage have confirmed Mormon denial of the Trinity (Articles of Faith, p.35).
It is a mysterious doctrine, but the Bible does teach that there is One God eternally existing in three Persons (Matthew 28:19). Because Mormonism holds a distinctly unbiblical view of God, the Mormon Church should be considered a religion distinct from Christianity rather than a part of it.
(Editor’s note: many of the references in our articles on Mormonism are Mormon publications, such as Mormon Doctrine, Articles of Faith, Doctrines of Salvation, History of the Church, Doctrine and Covenants, and so forth. Others are from the Book of Mormon itself, e.g., books such as 1 Nephi, 2 Nephi, and Alma.)
