Given my passion for anti-theism, I may not be an accurate representation of the typical atheist, but the fact remains that I know more than a vast number of believers about their own religion. (Although I admit that I'm lacking in non-Christian religious knowledge, but I know more about other religions than most Christians do.)
But the point is not that I'm an atheist, so I know more about religion; that's the wrong cause and effect. I think the more accurate way to phrase it is that I know more about religion, so I'm an atheist.
As the late, great Isaac Asimov wrote: "Properly read, the Bible is the most potent force for atheism ever conceived." -- Isaac Asimov
(More quotes like the above on my atheism quotes page.)
In short, reading the Bible in detail -- actually paying attention to what's written on every page, not cherry picking -- easily is the best tool out there for atheism. It's also a great tool for developing critical-thinking skills.
The more you know about religions other than your own, the more likely you are to realize how similar they all are. Native Americans believed in their creation myths just as passionately as you believe in yours now. Muslims are just as convinced that they're right as Christians are, and for the same reasons (ancient writings, personal "experiences," and community). It doesn't take much for a rational person to put two and two together to recognize that it's more likely that all current religions are false than only one is true.
A detailed knowledge of the Bible and a better understanding of the numerous world religions is key to organically creating an atheist. How ironic.
Here are some pro-atheism tools that any proactive atheist should feel fine giving any religious friends they have: