The argument that the Atheist most often makes, most often in public but also in private conversations, is NOT an argument that God does not exist. It is also not an argument that the Bible or the Koran or whatever holy book a particular faith adheres to is somehow incorrect or false. Instead, the most common argument of the mission-driven Atheist is that a person of faith cannot also be a person of reason or intelligence. In simpler terms, the atheist argues that if you are a Christian (for example) you are an idiot.
The argument that the Bible is *not* true, at least in an objective, testable way, isn't much of an argument when posed to anyone but the most strict biblical literalists. Most educated people of faith understand a holy book to be a very different document than a science book or a treatise on logic, a book that is understood differently than one understands science or even more subjective forms of writing such as history or journalism. This premise is easily testable--what reasonable person believes that snakes talk, or giant fish swallow men? If we allow that this is a small percentage of reasonable people and we also allow that a greater percentage of reasonable people also profess to be Christians or Jews or Muslims, we can deduce that many reasonable people of faith don't literally believe many biblical accounts.
In a recent episode of Real Time with Bill Maher, the host accuses a Christian guest of "cherry-picking" only the "good things" in the Bible. I am surprised that the guest didn't point out that Maher, in pointing out the bad things (as when he calls the Old Testament God a mass murderer--a characterization I can't disagree with) is also "cherry-picking." This should not surprise anybody--we do this as a matter of course all the time. Accepting or rejecting certain parts of a whole does not mean wholesale acceptance or rejection of the whole. This fallacy, though, allows Maher to accuse people of faith of hypocrisy or stupidity. The fact is, reasonable people of faith do not, on the whole, believe everything in the bible as literal truth. They just don't.
Similarly, ask a religious person to prove that God exists. He can't do it. He simply cannot. But there are plenty of people who believe in God. This point, then, is hardly arguable. The Atheist easily wins this. You cannot prove a negative.
So what the Atheist is really saying to the believer is that the believer is somehow less intelligent or, at least, misguided for professing religious faith. In simpler terms, I'm smarter than you.
The Atheist, then, is not making an intellectual argument so much as a social one. The Atheist doesn't want to associate himself with someone who possesses Keats' negative capability. The Atheist is not trying to convince you that God doesn't exist or that the Bible is bunk. He is trying to tell you that he thinks of you as a lesser being for having the audacity to believe in or find comfort in something that is not scientifically testable. The Atheist does not like you. Why? Because his myopic view of the world dictates that he is smarter than you. And he doesn't like to hang out with fools.