I talked to my brother last night, and he made an excellent point. It's a little bit nonsensical to consider whether or not Christianity is true before you decide whether or not God is real.
I think there are two sides to this. Certainly you could argue that taking God out of the context of a particular religion renders God much too fuzzy, much too hard to define, much too ambivalent to take seriously as a concept. After all, the differences between the God characters in major religions are pretty big. If all we're really saying is that there's some big power out there that we don't understand, that's not much of a claim. That's almost something Atheists could get behind, so long as we call the power something other than God.
The other side is that you can't really relevantly consider fine doctrinal points until you acknowledge that what you're arguing about it a real thing. It's one thing to point out inconsistencies in a book about a fictional character, and it's quite another to challenge the character of God. More to the point, no argument for Christianity would be cogent without the implicit assumption of God, and so no argument for Christianity will seem even reasonable to him who has not accepted that there is a God. It would be a bit like arguing about the finer points of evolutionary theory with someone who doesn't accept the scientific method as a valid framework. You might give air to some interesting points, but you're never going to make real progress. In fact, you may do more harm than good- any theory that presupposes God to support religion will strike the Atheist as absurd, and any theory that assumes there is no God to defeat Christianity will seem to the Christian to be nothing more than a close-minded attack.
I think this is the point on which so many Christian/Atheist discussions get stuck. Christians come to the table assuming the existence of God, and Atheists come to the table assuming there is no God. These are vastly different starting points, and it actually proves quite difficult to construct an argument about anything meaningful that works in both frameworks simultaneously.
I was definitely guilty of this in discussing my questions of Christianity with the girl I keep mentioning. She would enter the discussion with the assumption that God exists, and it is therefore rational to explain things in terms of their relation to God. I would enter the discussion with the assumption that God must be proven real (which upon further reflection, as I mentioned in The Christian View of Conviction, I've come to believe is impossible) before you could invoke his name in any argument. Naturally, chaos ensued.
I think you need to accept the POSSIBILITY of God before you can meaningfully talk about Christianity (or any religion, for that matter). Just some food for thought.