First, I feel like I need to address the comments about responding to Christians on their own turf.
Okay, I grew up heavily Xian. I read the Bible over and over and over. I'm sick of it, and I'm just not going to use it, even when responding to Xians. It is not part of my governing meme anymore, and I feel like using it, even as a response to an argument, is silly. I feel silly doing it. I will discuss some things from the Torah (Old Testament = Torah) with my husband, partly because he views it as a cultural resource, and as a source of understanding Jewish history. He is not a literalist, and is not particularly interested in converting me, so it is pretty neutral territory for discussion. And, well, since he's my man, and thus a badass of heroic proportions, he's always got something interesting to relate. But that pretty much the only conditions I will discuss anything biblically related. For me, it feels like discussing Greek or Roman myths ...
Mostly, I'm interested in discussing non-theistic forms of living, living morally, living happily, living in balance with the world and the rest of humanity. I'm not wasting my precious brain power devoting time to arguing with people about, in essence, whether the tooth fairy is supposed to fly through the air or walk. It's SILLY!
I have no objection to others using it as a resource for argument with Xians, but I feel that you do so at your own risk. I think that you undermine your rhetorical position by even acknowledging the legitimacy of their resource. It also invites continuing witnessing by said Xian, if they are of the evangelical bent. Which is annoying and time-wasting, because you have to pretend to be polite if they are someone you cannot offend with impunity. If you already have a very strong grounding in Biblical scholarship, I would not discourage you for using your knowledge to speak to an Xian on their terms, but I also would not encourage any atheist to start *studying* their holy text in order to make more effective arguments against Xianity.
If you really want to argue with a Christian, but want to do it without arguing on biblical terms, stock up on some research on Zoroastrianism; http://en.wikipedia.org/wiki/Zoroastrianism. You know, the one that Xianity is pretty much based on. Wow! *That's* some rich material for argument. Start talking about the beginnings of dualism and see their eyes glaze in horror in an instant! "What?!! The whole righteous battle between good and evil thing began with them?" Oh, and there's so much more..... let me tell you all about virgin births. It was the trendy thing to do at the time. All real religions began with a genUine virgin being pregnated by a god. Mary was not the first.