Far too many atheists attempt to shift the burden of proof entirely on to the theist by insisting that atheism is simply the lack of belief in god or gods. This move, however, cannot be sustained. The first question is this:
Do you lack belief for any specific reason or set of reasons?
If the answer is no, then your position is arbitrary, and not, by your own admission, grounded in reason.
If the answer is yes, then the second question is:
If you lack belief in god for a specific reason or set of reasons, then what are those reasons, and how do they justify you lack of belief? In other words, the atheist must defend the notion that his/her reasons justify a lack of belief. Therefore, the atheist *does* have a burden of proof to bear, and cannot entirely shift it to the theist.
The atheist has two choices: either concede that his/her atheism is not grounded in reason, and is therefore not worthy of rational discussion, or concede that it is grounded in reason, and thus share his/her burden of proof with the theist.