I became an atheist because in college I spent a lot more time around fundamentalists. The more time I spent around them and their rediculous justifications for thinking the Earth was 6,000 years old the more that it became apparent to me that if they could be that ignorant of why their beliefs didnt make sense, the same could be true of mine. So I investigated why I believed what I did and over time more and more of my religious beliefs failed to withstand the sort of scruity that I put them under and eventually I realized I couldnt call myself christian anymore because I didnt believe any of it.