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The error in most so called Christian religions, is that they do not hold the Bible in unique authority.
There is always an, "AND."
Roman Catholicism - The Bible AND holy tradition.
Mormons - The Bible AND the book of Mormon.
J,W.'s - The Bible, (their version of it), AND the writings of Russell etc.
Even in Judaism, they have the Torah, AND the Talmud, which is human in origin and yet carries equal weight to divine revelation.
Fundamental Christianity relies solely on the authority of the Bible.