The Greatest Show on Earth by Richard Dawkins. That book explained evolution in such an easy to understand way that it made perfect sense. Once I realized that evolution was true, Christianity could no longer stand. No creation = no Adam & Eve = no original sin/fall from grace = no need for a "ransom" or a "redemption from sin", and the entire basis of Christianity falls apart.
Not sure if what I believe now could be considered "athism" or not, but I sure ain't Christian.