(1)Some Christians refuse to see other Christians as Christians.
(2)And your god gets smaller as well.
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From your wording of "my" "god," I gather I am not wrong about you being a prime example of an "atheist or non-Christian" who cannot see or understand why many Christians view "Mormonism" as a "cult" that one must belong to for years and obtain the upper echelons status to become privy to their "well-guarded secrets."
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Christians have The Bible. That's it. And the way to heaven and the way to hell is summed up in two verses: John 3:16 for heaven. John 3:18 for hell. 
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It is not "denominational differences" that separates Christianity from Mormonism. It is Mormonism itself, with its "Revered Secondary Book" in addition to its "non-traditional translation of the Bible," that separates Mormonism from Christianity. 
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The Quakers and the Amish may be good examples of Christian lifestyles and beliefs that are very Christian.
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Mormonism is not. It is a "cult" of "secrets."