So... The interminable conversation with the misguided Christian is (at the time of this writing) still ongoing. I have now used that conversation as the basis for two complete posts, each describing an argument that would-be evangelists really ought to avoid. One of the problems with both of these arguments is that they deal with nonbelievers not on the basis of their actual beliefs and experiences, but on the basis of what the would-be evangelist believes those beliefs and experiences must be, which is generally based on a reading of the scriptures.
So, since I've just spent several pages insisting that would-be evangelists would get better results if they'd talk with real unbelievers about why they don't agree (rather than arguing as if we were the embodiment of the Straw Men they've been taught to expect), it occurs to me that this might be a good time to explain why I'm not a Christian. Also, it may make for a handy reference if I find myself in this sort of conversation again.
Naturally, if you're one of my Christian friends, you're welcome to skip over this. In fact, I'd prefer if you did. I don't have anything against Christianity per se, and I'll do my best not to condescend or insult, but still... reading a detailed description of why I don't share your beliefs could be kind of awkward. So if you do decide to read on, just bear in mind that this is not meant as a blanket condemnation of religious belief, and you weren't the intended audience anyway.
I was going to talk about my religious background, but to be honest that's just a distraction. So we'll skip that, and move on to why I'm not currently a Christian. My reasons can be divided into two basic categories: intellectual and spiritual.
On a fundamental level, Christianity doesn't make sense to me. This isn't any one element, unfortunately, but a whole combination of things. As a result, I'm not really sure where to start, so I guess I'll just pick an example and go...
The Doctrine of the Trinity was one of the first things that I tripped over when I first started putting things together: the belief that God the Father, Jesus (the Son), and the Holy Spirit are simultaneously three separate entities, and different manifestations of a single entity. While an all-powerful deity should be able to do things that would be impossible for lesser entities, this seems to me to pass beyond miraculous and into the realm of absurdity - something akin to creating a square circle. Also, there are several sections of the Bible ("My God, why have you forsaken me?") that only seems to make sense if Jesus is not part of God. However, accepting this doctrine isn't strictly necessary in order to be a Christian, so let's move on to more fundamental things.
Original Sin is another concept that I don't really accept. I've tried, perhaps unsuccessfully, to lay out my objections to the idea, but let's focus on my two main problems. First, if you read this literally, it means that all of humanity is being punished for a mistake made by a very distant ancestor. Second, the notion that we are subject to Earthly death, a sinful nature, and eternity in Hell as a "natural consequence" of sin seems completely incompatible with the idea of an all-powerful God. While it has a certain poetic appeal, the concept of original sin does not (in my opinion) provide a particularly accurate description of the human condition.
Penal Substitutionary Theory is the idea that Jesus' death on the cross paid the price for our sins. I learned the name decades after I came to reject the doctrine; my objection was that I could not - and cannot - see how the death of one man, two thousand years ago, could possibly make any difference to my sins. (This was something that confounded Dr. Pulliam also; a very great deal of his blog was devoted to examining various views of the issue.) This question is further complicated if you accept the Doctrine of the Trinity; at that point, you have God sacrificing Himself to Himself in order to satisfy His need for justice, which is just bizarre.
There's an additional problem which only occurred to me much later. The price of sin is an eternity of torment in Hell, right? But Jesus was somehow able to pay this - for everybody - in a couple of days of Earthly suffering? How does that work, again?
And, of course, my inability to accept the idea of Original Sin means that I don't really see any need for this sort of sacrifice to redeem people. That's not to say that we don't need forgiveness, just that we ourselves can repent, apologize, and try to make amends to the best of our ability. To me, that sort of personal attempt is a lot more meaningful.
There are some other issues floating around - for example, the Bible looks to me far more like a record of mankind attempting to communicate with the divine than the other way around - but that ought to be sufficient to illustrate what I mean when I say that Christianity as whole simply doesn't make much sense to me. Those aren't minor doctrinal quibbles that could be fixed with better discernment of the Scriptures; Original Sin and Penal Substitution are the core of Christian belief.
Spiritual (Revelatory / Emotional) Reasons:
I have felt the touch of the Divine. It came to me as a sense of transcendant calm, and peace, and renewal. It was also a sense of Presence, of something unseen but powerfully alive. It didn't seem to want anything from me, or have any particular message; it was just this unexpected and unasked-for gift of grace.
Considered in retrospect, I have no particular reason to believe that this was something done to/for me by an outside entity. Intense and unexpected as it was, it might have been nothing more than an aesthetic, emotional reaction. But let's assume, for the sake of argument, that it really was the touch of the Divine. I have no particular reason to believe that it wasn't.
I've experienced this twice. In both cases, I was fairly isolated: once alone, once with one other person. In both cases I was out in the woods of eastern Tennessee - the Cumberland Plateau area, which is overflowing with natural beauty. Both experiences came from being close to nature, without anything in the way of civilization to offer distractions. And both were... "meaningless" isn't quite the right world, but if there was some deeper meaning or higher purpose or life-altering message that I was supposed to take away from the experience, I missed it.
I've never felt anything like it in a church. Oh, the great cathedrals at Chartres and Notre Dame stirred a somewhat atavistic sense of awe, with their strange combination of deep forest and cavernous stone; but it was nothing like that sense of presence and peace that came to me in real solitude, with real trees around me.
I've never felt anything like it while reading the Bible - or any other book, holy or otherwise, for that matter. Not even close.
I've never felt anything like it among any group of people, no matter how kind or friendly or moral they were. Nothing.
So, putting it all together:
I'm not a Christian because Christianity doesn't work for me. It doesn't provide me with a particularly accurate or even useful way to examine the human condition; it doesn't offer any unique insights into the nature of the physical world. And insofar as I have any experience of divine or spiritual matters, it doesn't accurately describe those, either.
Again, if Christianity accomplishes any or all of that for you, that's fine. I'm not trying to argue that it's worthless for everyone. But please accept that what I've said here is... well, that these are my reasons. They aren't just excuses to cover my rebellion against God; if God exists, I genuinely don't perceive Him. (And even if He does, I don't see any reason to assume that Christianity accurately describes His nature.) Believe it or not, I really don't have a "Jesus-shaped hole" in my heart - or if I do, I can't find it, which amounts to the same thing.
So when I say that I can't be a Christian, this - all of this - is what I mean.
 I didn't give it much actual thought at the time. There didn't seem to be any need, and I was enjoying the experience too much to interrupt it.
 No, not rejected it. Missed it.