In the course of writing a blog post on my naturalistic worldview, I have composed an inconveniently long argument against theism (belief in a personal God). Rather than include it in the naturalism post, I've decided to post it first and link to it later. For whoever cares, there is now another argument against theism on the internet.
Disclaimer: I know that there are many answers to each of the objections I raise here, and there are important conceptions of God that are not addressed here. I don't think I have a knock-down argument against the existence of God, and I don't think I'm going to argue anyone out of theism. Please read this as an explanation of why I am an atheist, and why I think that atheism is a plausible position. Readers are welcome to challenge the argument presented here, but I don't want anyone to start out with the wrong idea.
This is my own version of a traditional argument. Science and history provide us with a couple of major streams of data, and I'll argue that a non-theistic worldview fits the relevant facts better than a theistic worldview.
Science provides us with an account of the history of life in which descent with random variation and the process of natural selection, along with other causal factors, have caused the patterns of variation, adaptation, and relatedness that we see among all modern organisms, including ourselves, from the first origins of life on Earth between 3 and 4 billion years ago (which scientists still don't understand), to Earth's verdant present. The history of our species' civilization yields a narrative in which each of the various human societies on every major land mass on Earth has developed its own mythology and forms of worship to suit its particular way of life- traditions which are, in some cases, partially embodied in and inspired by texts those societies hold sacred.
How does a theistic worldview accommodate these offerings of science and history? The account of human origins given by the theory of evolution seems to make a creator God superfluous. The task of creation can be pushed further back to the origin of life or even the origin of the cosmos, and while science does not yet (and may never) provide explanations of these events, just saying "God did it" is no better than posting a big question mark at those points in the timeline. This does not rule God out completely, but if God did create the cosmos, or life, by divine fiat, then why create a world where life would run on and on in such a wasteful, chaotic, and savage process as natural selection? Did a God of love determine that humanity would evolve through a process of adaptation in which the unfit are pared away by horrific deaths to end miserable lives, and their fit competitors would often meet equally awful ends a short time later? The history of life revealed to us by science is, to say the least, apparently incongruous with the notion of a benevolent, personal creator God. While God is not ruled out altogether, there doesn't seem to be any special place where God fits.
If God yet exists, there is a question of whether Christianity, Judaism, Islam, Hinduism, or some other religious tradition altogether can lead us to communion with the Creator. If it is Christianity, which Christianity is it? The same question applies to the others. If one of these traditions truly is inspired by God, and the others are counterfeits, how can we tell which is the real one? All of them seem to have equal claim to divine inspirations and miraculous interventions, and all of them seem to be overwhelmingly correlated with accidental factors such as geography and upbringing, rather than some special factor that sets one of them apart as a more credible option. Did God create the world in such a way that humanity would evolve and then develop a wild array of spiritual traditions, and then just inspire one of those traditions without telling everyone that it was the right one? Like the history of life on Earth, the history of humanity in particular seems incongruous with the notion of a benevolent Creator.
Does a non-theistic worldview have these kinds of problems? Natural history and human history have unfolded in a way that is perfectly consistent there being no God- at least the bits that we know about. What about the bits that we don't know? What about the origin of the cosmos, the origin of life, the nature of consciousness, and whatever other various gaps there are in our understanding of the cosmos? As mentioned before, these are things that we don't know, and may never know- things we may not even have the capacity to understand. But is it better to accept some ancient myth whose veracity I hope now seems very shaky, or is it better to own up to our (blameless) ignorance about these matters, admitting to and perhaps even celebrating to our place in a truly mysterious cosmos?
I offer one more consideration in support of a non-theistic perspective. We humanbeasts learn about the world by understanding new experiences in terms of what is more familiar to us, and what is most familiar to us from the very beginning is ourselves, and one another. Our most fundamental experiences, beyond the basic sensory modalities, are personal, social, and symbolic. The concepts in terms of which we understand these experiences are those with which we will first try to interpret the world at large, wondering who put the stars in the sky, why water chooses to run downhill, what it feels like to be a tree, and how to translate animal sounds into our languages. We anthropomorphize promiscuously, construing for ourselves a role in an enchanted world, and we as a species have only slowly and painstakingly come to know that the real world really doesn't revolve around us. I suggest that God, rather than being the ultimate spiritual insight, has been the grandest expression of our anthropomorphic mode of understanding. Like the other forms of anthropomorphism we have learned to withdraw from our understanding of the world, we will let God take its rightful place as a fondly remembered story as we proceed in our commitment to the truth.