Oh, it’s even worse than that! Joseph Smith said “I told the brethren that the Book of Mormon was the most correct of any book on earth, and the keystone of our religion, and a man would get nearer to God by abiding by its precepts, than by any other book.”
Critics of Joseph Smith, Mormonism, and the Book of Mormon use this to set up a false scenario, saying at least implying that Mormons are disparaging the Bible. This has led to people thinking that Mormons don’t even use or believe in the Bible. This is simply not true. Mormons revere the Bible every bit as much as they do the Book of Mormon. What Mormons believe is that the Bible, having passed through who knows how many hands of how many translators and writers over the past 2,000 years, inevitably has been changed, modified, mistranslated, and therefore isn’t exactly what was written down by the prophets and apostles. Mormons aren’t saying the Bible’s a pack of lies, we’re saying that it contains a some errors and there are a few things missing. A primary purpose of the Book of Mormon is to corroborate the Bible as a second witness and clarify things that may be confusing. Mormons use the Bible and Book of Mormon together. They’re even printed by the LDS Church as a single book with all sorts of cross-references. The LDS Church has official curriculum to accompany the Bible that every member studies. It would be hard to say that Mormons don’t use the Bible or revere it every bit as much as members of any other Christian religion.
So why did Joseph Smith say the Book of Mormon is the keystone of our religion and not the Bible? Because the Book of Mormon is the primary means of proving the LDS Church is true. Every other Christian church already has the Bible, and proving the Bible is true doesn’t help you decide whether to join the Catholic church or the Baptist church. But if you can find out for yourself that the Book of Mormon is true, then there’s only one option.