- I don't know what you're talking about
- I don't know why you would believe such a thing
A physicist will actually tell me what she's talking about and why she believes it. Funny thing: when she explains it, I can (with a bit of study) understand what she's talking about and, if she has good evidence, come to believe it myself.
The theologian, on the other hand, often bristles at these assertions. He can't tell you what he's talking about, and if he happens to say something meaningful (usually by accident), he can't or won't tell you why he believes it. Theologians will usually try to bury you in doubletalk and ambiguity, hoping you'll think it's all too complicated for your tiny little mind and just accept their authority. Newsflash: If you truly understand something, you should be able to explain it to a curious six-year-old, or, for the really super-complicated stuff like Quantum Mechanics, an ordinary college freshman (excluding those at Christian colleges, of course).
Atheism is not true just because Christianity bullshit, but Christianity is indeed bullshit. When atheists condemn theists (especially Christians and Muslims) for shifting the burden of proof, we're simply demanding they clearly answer the questions: What are you talking about? Why do you believe it? Anyone who tries to shake off the burden of proof does so only because they cannot or will not answer these simple questions.