This is an excerpt from an Ebon Musings article.

The argument goes something like this:
"If your religion is true, why did it originate in one portion of the earth at a specific time, rather than a universal revelation to all?"

The main topics covered (in detail) are:
Any deity which desired to be believed in would reveal itself to everyone, not just to a specific person, culture, race or nation.

If there is a reward for believing, it is fundamentally unfair that some would receive more and more reliable evidence than others.

If there is a punishment for not believing, it is fundamentally unfair that some would receive less evidence than others, or no evidence at all.

A religion which strongly reflects the beliefs of its time is more likely to be a product of its time than of revelation.

He goes into great detail - I'd ask that everyone, atheist or theist, read it.