For me the turning point was probably the problem of theodicy, i.e. why a supposedly All-loving, All-powerful and All-knowing God has created such a shitty place. That, and reading the Bible while asking questions, of course. Though I am an Agnostic Atheist, i.e. I don’t claim “there is no God”, but I just don’t really believe in one. Although I am rather fond of the idea that there is a God and he’s a frustrated teenager playing an advanced computer game (either that, or a cold AI running a simulation, which might already be over).