The short version: The problem of evil seems to me to be unanswerable. God scarcely ever appears to do anything, despite ample reasons for him to do so. The Bible is full of serious problems. Christians, despite supposedly being indwelt by the Spirit of God, don't appear to behave any better or show any more wisdom than everyone else. No evidence or arguments in favour of Christianity appear to be anything like as strong as these arguments against. Therefore, I quit.
A bit of background
I have, in at least some sense, spent pretty much all of my life to date as a Christian. I was baptized as an infant, which may or may not count for much but is about the most anyone can say about that part of their life. I don't remember any time in my childhood when I didn't believe in God and (more or less by default) assume that Christianity gave a good account of him and of his relations with humankind. My parents gave up Christianity before I can remember, but my first couple of schools were to some extent Christian ones; perhaps that's why I made those assumptions.
While I was at secondary school, I started taking these matters much more seriously; I fell in with a group of evangelicals and at their encouragement began to do such things as attending church, reading the Bible, praying frequently, and the like. For those who consider it important to isolate a first definite "commitment to Christ", that's when mine was. And for those who consider it important that one's commitment be continually reaffirmed and put into practice: there was until very recently no break in my Christian faith, practice and commitment. (Plenty of minor fluctuations, of course.)
But through all this, I have been nigglingly conscious of one awkward little fact: I never had much in the way of reasons for accepting Christianity, and the more I thought about the matter the more reasons for rejecting it I found. I mostly dealt with this by not thinking about it. (About reasons for and against Christianity, I mean; I gave a lot of thought to Christianity itself and to its consequences for my life.) Thus I attempted to exercise the theological virtue of faith, in one of its aspects: continuing to believe what you've committed yourself to, despite difficulties. But, well, there comes a point at which trying to ignore the problems ceases to be properly called "faith" and begins to be stupidity or mere stubbornness. And, being by nature a thinking sort of person, I wasn't terribly good at not-thinking-about-it, and the problems mounted until they reached and passed that point.
So, for about the last year and a half (as of June 2006) I've been systematically trying to get my mind into order, listing every good reason -- and plenty of not-so-good reasons -- I could find for and against accepting the truth of Christianity, and then considering them as fairly as I could, with much thought, reading and prayer. (And yes, I know that Christianity isn't just a body of doctrine; but it includes a body of doctrine, or at least the only versions I consider worth calling Christianity do, and it's the doctrine rather than the practices, institutions and attitudes that I found troublesome.)
The result: the reasons against far, far outweigh the reasons for. I make no claim about other people, whose evidence may differ from mine in all sorts of ways; but on the basis of what I know and have been able to find out, I can't remain a Christian with any sort of intellectual integrity.
A little more detail
Above, I listed very briefly some of the reasons why I cannot continue to accept Christianity. I'll say a little more about them, though it's no part of my intention to convince anyone else to agree with me nor to give a complete account of my thinking on any point.
The problem of evil. Imagine what sort of world you'd expect perfect goodness and infinite love, coupled with infinite intelligence, knowledge and power, to produce. Does it look like this one? No. (This is accepted even within Christianity; hence the idea of "heaven" or "the kingdom of God" or whatever you want to call it, where "God himself will be with them; he will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning nor crying nor pain any more, for the former things have passed away".) Even making allowance for the likely brokenness of our expectations, this seems to me to be a formidable problem for any sort of belief in a perfectly good and infinitely powerful God. There have been many attempts to deal with, or sidestep, this challenge; I think I've looked carefully at all of them. They're all hopeless.
The apparent apathy of God. First of all, my own experience. I have never found that my prayers were "answered" any more than I'd expect them to be by chance. I have never had anything I've recognized as an encounter with God. In short, for all I can tell directly, the world works just about exactly as I'd expect it to in the absence of God.
This, of course, is just one person's experience; but it's not so very unusual. And, so far as I have been able to tell, people with experiences very different to mine also occur with about the frequency predicted by chance. Apparent exceptions seem better explained by self-deception (to which we are all subject, alas), deliberate other-deception, and mere coincidence, than by supposing that Christianity is right and God distributes his favours at random, arranging odd little coincidences much more willingly than (say) curing terrible diseases.
The Bible. There's plenty of good stuff in the Bible. (The passage from Revelation that I quoted a few paragraphs above, for instance.) But there's also a dismaying amount of bad stuff. Whatever some liberal Christians may say, Christian belief is founded on the Bible; I think it's a foundation of sand.
Open a Bible at random and have a good look at the pages before you. Most of the time, you'll find something troubling: bloodthirsty vengeance on Israel's national enemies, prophecies that seem to have gone unfulfilled and passed their use-by date, bizarre exegesis of earlier scriptures, laws supposedly issuing from the very mouth of God that don't make any sense, casual acceptance of slavery, apparent incompatibilities with other things in the Bible or in well-established Christian belief. I have never been a fundamentalist nor considered fundamentalism necessary to Christianity; it's no big deal if the Bible is a human as well as a divine document; but the level of trouble I find seems to be pretty much what I'd expect if it were only a human document, and not at all what I'd expect if anything remotely like "all scripture is inspired by God and useful for teaching the truth" were correct.
Christian behaviour. There are those who would claim that Christians behave much worse than others, typically citing some nasty incident of narrow-minded bigotry. I think this is wrong; not being a Christian is (alas) no protection from narrow-minded bigotry, still less from other vices of which Christians aren't so often accused. But Christians don't seem to be notably better or wiser than others, either. Which isn't a problem in itself -- no one ever guaranteed that being right about God would lead to behaving optimally -- except that one of the things Christianity says is that Christians have the Spirit of God living inside them, leading them into the truth and reforming their character. Sure, there are instances of dramatic conversion leading to release from destructive behaviour patterns, but where are the perfect or near-perfect everyday lives that ought to be lived by people indwelt by God himself?
Some tedious corrections in advance
I would like to dispel a few possible misunderstandings before they arise. You needn't, of course, believe me, but so far as I am able to tell ...
... None of this is the result of having been hurt by any Christian institution or its (official or unofficial) representatives. I've had a few disagreements with vicars and plenty with individual Christians, but nothing to cause lasting offence or injury (to me, at least, and I hope not to them either). Christianity has, on the whole, been good to me.
... None of this is the result of a wish to live some sort of lifestyle forbidden or discouraged by Christianity. My moral convictions are (so far, at least) basically unaltered, except as regards some specifically religious observances that I never found particularly disagreeable, and I have no plans to start behaving in ways that Christians would find objectionable -- other than that trifling matter of not believing in God, of course.
In short: so far as I can tell, all that's happened is that I have ceased to find the doctrines of Christianity credible, as a result of the nearest thing I can manage to rational investigation. Please feel free to speculate about ulterior motives and the like, but please also forgive me if I'm skeptical.
Atheist? Agnostic? Or what?
Unfortunately, these terms (just like "Christian", I suppose) have often been subject to tactical definition, where everyone defines them in whatever way they think will be most uncomfortable for their opponents in debate. So, rather than playing that game, I'll just set out my current position. It may, of course, change.
I'm really quite sure (let's say, comfortably over 99.9%) that Christianity, at least in any form that deserves the name, is wrong.
I'm fairly sure (let's say, well over 95% and probably over 99%) that the universe is not being overseen by a perfectly good and infinitely powerful being, nor any very close approximation to one. (The problem of evil suffices, all on its own.)
I have no idea whether the world was created. The mere fact of its existence is a bit of a mystery, but not one that's helped by postulating a creator: the fundamental puzzle is "how come there's anything?", and a creator needs explaining no less than a universe.
I'm adopting materialism as a working hypothesis; it seems to hold up pretty well, but the world is strange enough that I'm not terribly confident about it.
I am not completely certain of anything. (In this domain or in any other.)
I am, or at least I aim to be, always open to having my mind changed. (I was pretty non-open on this particular one for a long time, thanks to that "faith" thing, but I aim to be more flexible in the future if real evidence or arguments should come along that I've missed.)
A few other notes
I would hate to lose any friends over this. To my Christian friends and acquaintances: please don't think I think any the less of you merely because I disagree with you. Please feel free to try to convince me that I've made a terrible mistake. (If you do, then I shall feel correspondingly free to try to convince you that I haven't, though I'm not particularly the evangelizing -- dysangelizing? -- sort.) And by all means pray for me if you wish.
I'll be mortified if anyone's led by this to give up being a Christian. But pleased if anyone who's in a situation at all like mine is led to think things through as I did, regardless of what conclusion they reach. Just as Christians often say that conversion isn't something one person does to another but something that happens in an individual's heart through the working of the Holy Spirit, so I say that neither conversion or deconversion has much point unless it's truly your own.
I'm not bitter. To my clerical friends and acquaintances: please don't think I resent your church, or any other. Nor do I resent God; I just don't think he's there at all.
I'm not afraid of ending up in hell, though of course I admit the theoretical possibility. A deity who causes, or even permits, a large fraction of the human race to end up in eternal torment or anything like it is unlikely to deserve worship.