I have, on many occasions, been asked questions that carry a pile of false assumptions, and it is much more interesting to try to address the underlying misconceptions than to attempt to answer. For example, "Why did you stop being a Christian?" An important error there is the assumption that the rationale I gave when I decided I was no longer a believer is the same as the reasons I would give today. "Why are you not a Christian?" is a better question, but I think there is an important issue that still needs to be addressed before launching into a discussion of this topic - and a discussion it must be. There can be no short answer, because the question "Why are you a Christian?" rarely admits of a concise response, either. I think religious people in general do have a wide variety of reasons they believe. To stop believing requires much more than just one of these foundations of belief being challenged or even overturned.
As a side note, this is partly why I disagree with those atheists who say that arguments with the religious can never bear fruit, just because no one changes their mind just as a result of one conversation. No, it takes many conversations, with many other thinking beings than yourself. The general reluctance of nonbelievers to respond to proud, public affirmation of belief in absurdities surely must help to prop up the faith culture.
So, because I, like most Christians, was a believer for many reasons, not just one, it was also not for only one reason that I one day found I was no longer able to believe. It can be a difficult process for the apostate to deconstruct after the fact. The thinking person's loss of faith, by necessity, is not a conscious process. It is no secret that religious belief, the more detailed and counterintuitive it is (American Christians may read that as "the more fundamental and evangelical it is"), requires active maintenance. Because worship and prayer and church groups are all part of this act of rationalization and reinforcement via groupthink, a critical mind can find its path away from belief through immersion in the very culture that is designed to prop it up. I am lucky in that my wariness of self-deception and desire for rationality served me well in this regard. I do not claim credit, for I did not choose these traits - perhaps I am merely fortunate that there were books in my home and my preference for solitude and lack of access to video games led to enough reading to cultivate my mind. (Ironically, C.S. Lewis and other apologetics featured prominently in this reading.)
I hardly intend to enumerate here my objections to faith, as more eloquent writers than I have tackled the subject at length in recent years, and they succeed in filling such tomes as "God: The Failed Hypothesis" and "God Is Not Great", which for all their concise language and clarity of expression, fail to be nearly as short as a blog post. While my most mature and most central complaint, I think, would be over the lack of a valid epistemology for generating any beliefs about the supernatural, it recently occurred to me that part of my gradual process of rejecting Christianity was a most generous and unintentional gift from a speaker at a youth group meeting.
"God answers prayers," the speaker said with conviction. Slowly and deliberately, ostensibly to denote profundity, he continued: "Either 'yes', 'no', or 'not yet'." (I mean no offense to any particular youth pastor or guest, as I have forgotten who delivered this platitude.) In retrospect, I am struck with horror at how such appalling stupidity was so frequently and solemnly inflicted on me at such meetings. My reaction at the time was not nearly so severe. I was simply made very uncomfortable by this pronouncement, as it did not take any great mental acuity to note that if this man's claim were true, then the world we live in would be, in that regard, indistinguishable from one ruled by a god completely indifferent to prayer. I decided the speaker was wrong, but I was as of yet unaware that my faith was diminished. The wall did not, and never did, fall at the sound of a trumpet - it took the work of many to tear it down.