This is a very basic look at Mormonism, also known as the Latter Day Saints. Key falsehoods about Mormonism from an ex-member can be found here.
In studying major world religions, it becomes clear that the founder of Mormonism, Joseph Smith, plagiarized Islam, as can be seen in this comparison. To this we can add the male-centric nature of both, as shown here and here. Both view heaven and earth as places of perpetual sexual indulgence for men, and perpetual submission for women.
As detailed in other chapters here, the Bible passes all tests of authenticity and integrity. No such support exists for the Book of Mormon; its authenticity depends completely on the integrity of Joseph Smith and his alleged witnesses. But Smith was proven to be a liar when he claimed to have translated the golden plates from reformed Egyptian
. He took the translation to be verified by a Prof. Charles Anthon, who concluded that this was merely a hoax made up by Smith, but Smith claimed he said it was authentic. Some of the witnesses of the golden plates later disavowed ever seeing them and were condemned by Smith. So Smith and his witnesses have been discredited; see also this source.
The subtitle of the Book of Mormon is Another Testament of Jesus Christ
. This is countered by 2 Cor. 11:14, which says that all other teachings than what the original apostles taught are false. The chapter headings sometimes make reference to America, but it never appears in the text, and there is no way to identify it from the vague descriptions of places and geography it contains. Large sections of text are taken verbatim from the Bible, but out of context and confused. Over all, the Book of Mormon contains the writings of one who is described clearly in Colossians 2:18-19.
Concerning priesthood, Mormonism teaches two priestly offices: Aaron and Melchizedek. But Hebrews 7:12-17 shows that the Aaronic priesthood was replaced by the Melchizedek priesthood, and that Jesus holds that position forever. No other priests are needed or qualified. Only those who are in Christ are priests (1 Peter 2:9), which includes all true believers. Mormon priests simply have no authority.
We must be discerning. For example, Mormons will say they believe that salvation is found only in faith in Jesus and that there is one God. But the meanings of those words need to be specified, since their church teaches that there are in fact millions of gods running their own planets. This article quotes their president Hinckley: As God is, man may become. We believe in eternal progression.
Salvation for them is faith plus works, and thus never guaranteed. Mormonism is indeed a different gospel
(2 Cor. 11:4, Gal. 1:6). There is a more thorough examination of Mormonism here.