Since a couple of readers have been startled at the speed of my volte-face from "christian" to "atheist", I should probably clarify what I mean by the latter.
I no longer see any reason to believe in God. That doesn't mean there isn't a God (or even that there's no reason to believe in one, since obviously I'm not infallible), but -- given the depth of thought I've applied to the area over the past two decades -- it seems to me to mean there's no God who's relevant to me. Absence of evidence is not evidence of absence, but Occam's Razor suggests that the null hypothesis should always be preferred in cases of ambiguity.
I see no reason to believe in God; therefore I don't believe in God; therefore I'm calling myself an atheist. Although I'm claiming no certainty in the matter, using the term agnostic would, in my view, just be pussyfooting around. (In a sense, and I accept that this bit isn't strictly rational, I feel as if I've given God enough benefit of the doubt already.)
That said, I don't like the label much. It defines a philosophy by an absence, and tacitly in opposition to an assumed norm, which is never a brilliant start. I might call myself a humanist instead, if I didn't find the British Humanist Association so irritating. Mind you, I claimed that label when I was a christian too, so maybe I'd be better off keeping it.
The alteration I'm finding most difficult to adjust to is, at this point, the absence of objective truth. I'm not talking about observable fact and the system of scientific knowledge which has been empirically constructed upon it, which is a separate epistemological category. Alpha Centauri may very well be 4.37 light years away, and I'm very happy to accept that as true based on the centuries of astronomical observation which underpin it... but really, it's not fundamental to my world. The figure could be discovered to be wrong by, ooh, anything up to about 20% and I'd still approach questions about human life and its place in the universe in much the same way.
Up to now I've been in the habit, sometimes consciously and sometimes not, of thinking in the face of a complex moral, philosophical or metaphysical issue, "Well, this is what I think, and I think I have good reasons for it, but we'll probably never know the truth -- only God knows that." In the absence of that final clause, the penultimate one breaks down -- if there's no God, then I've no basis for believing that there is a "truth", at least pertaining to any concept or phenomenon not resolutely physical.
Is Shakespeare really better than Marlowe? God knows. Do mathematics reflect a fundamental reality, or are they just a human construct? God knows. Do human beings have "free will" in any real sense? God knows.
Is murder always wrong? God knows.
That phrase used to be a reassuring one. Now it's just rhetoric. It turns out -- and I've always known that this was a standard atheist point of view, but suddenly to be living it is a horrible body-blow -- that there are no absolute truths. We make our own, as best we can, from the materials available, and if ours don't agree with someone else's... well, one may be better than the other according to certain criteria, and if we accept those criteria we may wish to adjust our own views accordingly, but there's no real question of one person being objectively right and the other wrong.
When you believe there are such "rights" and "wrongs", even if they're essentially unknowable in this lifetime, simply thinking they're there in the mind of God makes a radical difference to how you approach this whole business of thinking.
For the moment, until I have a proper framework to hang all this on, I have to accept provisionally that: 1. Attempting to follow a system of ethics is a valuable habit, certainly for society and possibly also for the individual; 2. Present-day liberal Western society, into which I happen by sheer fluke to have been born, has on the whole made pretty good assumptions about what make for worthwhile values.
If this sounds pretty feeble, well, yes. I'm working on it.
In fact, I don't think many of my previous values were directly derived from my faith, although I've tried with varying levels of success to claim that they were. My pacifism and vegetarianism always needed some fancy footwork, for instance, and it seems to me that there's a sounder foundation in humanism for justifying the view that murder is always wrong, than in what's necessarily an idiosyncratic interpretation of christianity.
All of this is a work in progress, obviously. Next time I may discuss why I still think Richard Dawkins is an utter tool.
 I say "in this lifetime" because this idea rather coloured my view of the afterlife. I saw Heaven as much as anything as a place where the unknown became knowable, all truths were laid before us for our contemplation, and where I could watch the future history of humanity unfolding on fast-forward like the greatest SF epic ever filmed. I bloody miss that.