Why are all Christians atheists?

This is the same argument that you hear a lot from Sam Harris and I think it is deeply flawed.The problem with it is that Christians or Moslems are not atheists with respect to other Gods due to the basic absurdity of said Gods. They are atheists with respect to other Gods because their holy books tell them to be.Zeus' existence is not considered false because the concept of zeus is absurd. Zeus' existence is considered false because the Bible says that there is only 1 God.There is nothing to be gained by calling religious people hypocrites because they don't accept the same rational for the rejection of other Gods. The argument fails because they are not rejecting other Gods for rational reasons, but rather religious ones.

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Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

I agree with you with the way you phrased it but the core question from the OP seems different than what you are talking about. The hidden implication in that question is that we are talking about hypocrisy. But they don't have disbelief in those Gods for the same reasons. ("I can't imagine/its not possible to not believe in God")They have disbelief in those Gods because they believe that their particular holy book instructs them to have this disbelief.This is why the argument fails with fundamentalists. They don't necessarily reject other deities on their base absurdity. They reject them because THEIR God says to.

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Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

I agree. But a fundamentalist is not going to see it that way. They are often going to cite some ethereal "fate" argument (i.e. I used to be into drugs, alcohol, satanism) for why they found their particular God and the exclusivity just gets snowballed in.It is not like all the pantheon of Gods that have ever existed were laid out in front of these people and they arbitrarily chose one.I just think that you are not going to find a meat in a discussion surrounding this challenge to religious people. Their rejection of other deities is not based on reasonableness or objectivity.

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Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)