There have been several moments during my life that particularly influenced me towards atheism.
One of these was when I first started learning about science. Scientific explanations seemed to me to be much more satisfying and believable than those of the bible. For a while I thought that this was because the people who originally wrote down the bible were not sophisticated enough to understand the science so it was written down in terms they might be able to understand on some level - that the bible was a simplified version of the truth that we would eventually figure out for ourselves. I had precedents for this concept - I had computer manuals which give strict instructions like "don't remove the floppy disk from the drive while the light is on" but I later discovered that you can remove disks during the time between when the save operation completes and the light goes out (the instruction in the manual was less efficient and less accurate but easier to understand and more difficult to screw up).
The second of these moments was probably when I was told the story of "doubting Thomas". Nobody ever seemed to be able to explain to me exactly why being skeptical was a bad thing (other than the usual "God works in mysterious ways" rubbish that I always found totally unconvincing).
The third was probably when I tried to read the Old Testament. This must have been during secondary school as well. I got as far as the story of the Tower of Babel. In the past when I heard this story it had been presented as a kind of "just so story" about the origins of the different languages. But when I read it for myself I realized that this God was a real dick! Rather than celebrating the great achievements of his creation, he scrambles their language out of fear and jealousy! ("...now nothing will be restrained from them, which they have imagined to do. Go to, let us go down, and there confound their language..."). At that point I was so disgusted I gave up.
The fourth was at university when (while debating a Christian friend) I came across a site called "Who Gives a Fish" (which now longer seems to exist). This linked to some extremely compelling arguments that all of Christianity is bunk. After reading that (and failing to find the refutations to it that I would expect to find if it were false) I no longer had any desire to be infected with this particular meme.
That I ended up an atheist despite my religious upbringing seems to me to be quite damning evidence against God - surely if there were something to it at least the people who started off religious would stay religious.