One of the more convincing arguments for atheism...

... for me, anyways, was the size and scope of everything that has nothing to do with us. Statements like "If the whole history of the universe were as long as your arm span, you can eliminate the entire history of humanity with the scrape of a fingernail." (I can't find the official reference, but it's used in the irrelevant introduction to this Ruby talk, until I can find the real one).

If God existed, and cared at all about us, why would he also create so much universe, and have it exist for so much time?

Even as someone who already feels this way, this graphic surprised me:



You can't see it, but to the left is our Sun. You really need to see the original image, and note that the Sun is represented by 1 pixel (the image itself is 10,173 pixels by 2500).

Also remember that the Sun contains 99.99% of the matter in our galaxy.

In a related note, Pharyngula points us to some video added to Carl Sagan (kind of) on the matter.