It is in the area of the doctrine of God where the greatest and most important differences prevail between Mormonism and Evangelical Christianity.
Orthodox Christians (Catholic, Orthodox, and Protestant) are monotheists who believe that the Trinity: Father, Son, and Holy Spirit are one God in 3 persons- three distinct persons who together are one being- God. While Mormons in their official ‘Articles of Faith’ declare: “We believe in God, the Eternal Father, and in His Son, Jesus Christ, and the Holy Ghost,” sounds like what other Christians believe about God; in reality they teach that the ‘Godhead’ is a triunity- 3 separate Gods unified in attributes and purpose:
“Though each God in the Godhead is a personage, separate,and distinct from each of the others, yet they are “one God’ meaning that they are united as one in the attributes of perfection. …and yet they are three separate and distinct entities. Each occupies space and is and can be in but one place at one time.”-Mormon Doctrine, page 319
Christian theologians view the Mormon definition of their ‘Godhead’ as tantamount to being polytheistic. Mormons do not like being called polytheists and claim that their belief in their Godhead as ‘one’ God is monotheistic. They also teach that God the Father is above all else, everything and everyone is subordinate; including The Son and the Holy Ghost. That ‘Subordination’ is considered to be heretical by all orthodox (Catholic, Orthodox, and Protestant) Christian churches.
Interestingly, if one does a complete study of the theology of God in their Book of Mormon, it mostly supports the orthodox teaching of the Trinity rather than the Mormon teaching on the Godhead as a ‘triunity.’ Though within Ether (a book in the Book of Mormon) a Sabellian (oneness) theology of God is supported. Also a heresy but the exact opposite of the Mormon Triunity teaching. So one is tempted to say: when it comes to the teaching and theology of God, the Book of Mormon vs. The Mormon Church?
For a good understanding of the Evangelical Christian view on The Trinity, see: Grudem, Systematic Theology pgs 226-261
-“Mormon Doctrine“, by Bruce R. McConkie (Salt Lake City, Utah: Deseret Book Co., 1979, 856 pgs) An important work, but not an official statement of LDS Church doctrine.
-(D. & C.)”Doctrine and Covenants” (Salt Lake City, Utah: LDS) One of the LDS books of scripture.
-“Book of Mormon” (Salt Lake City, Utah: LDS) One of the LDS books of Scripture.
-“Pearl of Great Price” (Salt Lake City, Utah: LDS) One of the LDS books of Scripture.
-“Inspired Version” of the Bible ( Independence, Mo.: Herald House) Printed by the Community of Christ (RLDS Church).
-“Systematic Theology“, by Wayne Grudem (Leicester, Eng.:Zondervan Pub., 1994, 1264 pgs.)