The following is a brief overview of the beliefs of Mormonism (The Church of Jesus Christ of Latter-Day Saints) believe, along with what the Bible really teaches, printed among the many articles and resources in the back of the ESV Study Bible (posted with permission).
1. Apostasy and Restoration
Mormons claim that “total” apostasy overcame the church following apostolic times, and that the Mormon Church (founded in 1830) is the “restored church.”
If the Mormon Church were truly a “restored church,” however, one would expect to find first-century historical evidence for Mormon doctrines like the plurality of gods and God the Father having once been a man. Such evidence is completely lacking.
Besides, the Bible disallows a total apostasy of the church (e.g., Matt. 16:18; 28:20; Eph. 3:21; 4:11-16), warning instead of partial apostasy (1 Tim. 4:1).
Mormons claim that God the Father was once a man and that he then progressed to godhood (that is, he is a now-exalted, immortal man with a flesh-and-bone body).
However, based on the Bible, God is not and has never been a man (Num. 23:19; Hos. 11:9). He is a spirit (John 4:24), and a spirit does not have flesh and bones (Luke 24:39).
Furthermore, God is eternal (Ps. 90:2; 102:27; Isa. 57:15; 1 Tim. 1:17) and immutable (or unchangeable in his being and perfections; see Ps. 102:25-27; Mal. 3:6). He did not “progress” toward godhood, but has always been God.
Mormons believe that the Trinity consists not of three persons in one God but rather of three distinct gods. According to Mormonism, there are potentially many thousands of gods besides these.
However, trusting in or worshiping more than one god is explicitly condemned throughout the Bible (e.g., Ex. 20:3).
There is only one true God (Deut. 4:35, 39; 6:4; Isa. 43:10; 44:6, 8; 45:18; 46:9; 1 Cor. 8:4; James 2:19), who exists eternally in three persons—the Father, the Son, and the Holy Spirit (Matt. 28:19; 2 Cor. 13:14).
4. Exaltation of Humans
Mormons believe that humans, like God the Father, can go through a process of exaltation to godhood.
But the Bible teaches that the yearning to be godlike led to the fall of mankind (Gen. 3:4ff.). God does not look kindly on humans who pretend to attain to deity (Acts 12:21-23; contrast Acts 14:11-15). God desires humans to humbly recognize that they are his creatures (Gen. 2:7; 5:2; Ps. 95:6-7; 100:3). The state of the redeemed in eternity will be one of glorious immortality, but they will forever remain God’s creatures, adopted as his children (Rom. 8:14-30; 1 Cor. 15:42-57; Rev. 21:3-7). Believers will never become gods.
5. Jesus Christ
Mormons believe that Jesus Christ was the firstborn spirit-child of the heavenly Father and a heavenly Mother.
Jesus then progressed to deity in the spirit world.
He was later physically conceived in Mary’s womb, as the literal “only begotten” Son of God the Father in the flesh (though many present-day Mormons remain somewhat vague as to how this occurred).
Biblically, however, the description of Jesus as the “only begotten” refers to his being the Father’s unique, one-of-a-kind Son for all eternity, with the same divine nature as the Father (see note on John 1:14; cf. John 1:18; 3:16, 18; see also John 5:18; 10:30).
Moreover, he is eternal deity (John 1:1; 8:58) and is immutable (Heb. 1:10-12; 13:8), meaning he did not progress to deity but has always been God.
And Mary’s conception of Jesus in his humanity was through a miracle of the Holy Spirit (Matt. 1:20).
6. Three Kingdoms
Mormons believe that most people will end up in one of three kingdoms of glory, depending on one’s level of faithfulness. Belief in Christ, or even in God, is not necessary to obtain immortality in one of these three kingdoms, and therefore only the most spiritually perverse will go to hell.
But the Bible teaches that people have just two possibilities for their eternal futures: the saved will enjoy eternal life with God in the new heavens and new earth (Phil. 3:20; Rev. 21:1-4; 22:1-5), while the unsaved will spend eternity in hell (Matt. 25:41, 46; Rev. 20:13-15).
7. Sin and Atonement
Mormons believe that Adam’s transgression was a noble act that made it possible for humans to become mortal, a necessary step on the path to exaltation to godhood.
They think that Christ’s atonement secures immortality for virtually all people, whether they repent and believe or not.
Biblically, however, there was nothing noble about Adam’s sin, which was not a stepping-stone to godhood but rather brought nothing but sin, misery, and death to mankind (Gen. 3:16-19; Rom. 5:12-14). Jesus atoned for the sins of all who would trust him for salvation (Isa. 53:6; John 1:29; 2 Cor. 5:21; 1 Pet. 2:24; 3:18; 1 John 2:2; 4:10).
Mormons believe that God gives to (virtually) everyone a general salvation to immortal life in one of the heavenly kingdoms, which is how they understand salvation by grace. Belief in Christ is necessary only to obtain passage to the highest, celestial kingdom—for which not only faith but participation in Mormon temple rituals and obedience to its “laws of the gospel” are also prerequisites.
Biblically, however, salvation by grace must be received through faith in Christ (John 3:15-16; 11:25; 12:46; Acts 16:31; Rom. 3:22-24; Eph. 2:8-9), and all true believers are promised eternal life in God’s presence (Matt. 5:3-8; John 14:1-3; Rev. 21:3-7).
See also the post on the beliefs of Jehovah’s Witnesses.
Here is a video of evangelist Aaron Shafovaloff talking to Mormons in Salt Lake City about whether it was possible God the Father could have been a sinner in a past probation: