When I first became an atheist, it was of the sort that people call 'weak atheism', and some would even have referred to it as agnosticism. Now that I've had some time to become comfortable with the idea that there aren't any deities, I've moved from the position that a god is a possibility simply lacking in evidence, to the position that gods really are pretty unlikely, and almost surely don't exist.
In a particular essay I wrote shortly after becoming an atheist, I summarized my position on deities and souls as follows.
To clarify my position on religious matters at the time of writing this essay, I'm not absolutely one-hundred percent certain about anything. However, I'm about as sure that the Earth is a globe that orbits the Sun as I am that the Bible was written by people, and that a God as presented in the Bible doesn't exist. I'm not as certain that no type of divine being exists at all. I don't see an absolute reason why there would have to be one, but that doesn't mean that there isn't one, or that a super powerful being didn't come into existence after the universe did. I'm also open to the idea that we have souls and will experience some type of afterlife. So, I may not buy into the arguments of Christianity, anymore, but I haven't rejected a spiritual aspect of the universe, altogether.
That certainly sounds sensible. So what's changed?
To start off, maybe I should begin with discussing certainties. In a philosophical sense, we can't be 100% sure of anything. There's always some small finite chance that whatever we think we know is wrong. The common arm chair philosopher argument is to ask how we can be sure that everything we know isn't just a dream, or a hallucination. Practically speaking, however, some scenarios are just so unlikely that their probability becomes infinitesimally small. So, while we admit that in a philosophical sense they have some finite probability, we live our lives as if they're impossible. No one goes to bed at night worried that gravity will quit working and that they'll wake up in space. So, when I wrote above that deities almost surely do not exist, it's only in that philosophical sense that I grant that they might.
Let me digress one more time before getting to the main point. Let me explain just why I was religious before I became an atheist. It really had very little to do with evidence, and very much to do with emotion and tradition. I'd been brought up going to church, and having authority figures tell me over and over how important it was to be religious, and how important it was to have faith. Even certain parts of the Bible stress how important it is to have faith without evidence (recall the story of doubting Thomas from John 20:29, or Jesus being tempted by Satan and replying that you shouldn't put God to the test from Matthew 4:7). I think the following comment I made on a blog during a discussion when I was still Christian shows just how much I valued faith over evidence.
I don't think it's a sign of personal weakness to believe in God. Knowing all that I know about science, it takes a lot of faith to accept the Bible. To me, that's more of a strength than a weakness.
I knew all along that I didn't have much evidence to support my religious beliefs, but I didn't let it shake my faith. In fact, the only 'evidence' I had for the divine were a warm fuzzy feeling that I assumed to be God's presence, and a very minor miracle that I personally witnessed (an object I'd lost turned up in a place where I was positive I'd checked very thoroughly). I knew all along that both of those forms of evidence were very shaky, and could very easily be explained through non-supernatural causes. I counted them as evidence because I wanted to believe, not because I thought they were strong evidence.
The reason I bring all this up is to reinforce that there really is no strong evidence for the divine. I've covered this in much more depth in other essays, but even when my faith was strong and I had no reason to doubt any evidence supporting a deity, I just didn't see it.
When I first became an atheist, it was through a rational approach. I recognized how little evidence there was for gods, and I realized I was a Christian mainly through accident of birth, and didn't have any real reason to choose it over any other religion, so I was left with atheism as the only honest choice. But those emotional reasons that kept me a Christian for so long were harder to shake than any logic. Remember, the type of god that most Christians believe in isn't the fire and brimstone version from the Old Testament, it's the 'God is love' version you learn in Sunday school. I didn't like to lose that eternal protector, and I wasn't yet ready to give up the promise of an afterlife, so that's why I clung to a god as still being a possibility, even though there wasn't any evidence.
Now, let's leave emotion behind. Even if Christianity wasn't the result of Yahweh intervening with his creation, it still had to come from somewhere. One interesting observation is that nearly every culture has a religion, nearly all of which include deities. It's pretty unlikely that religion has been invented out of thin air independently in each of those cultures. There are a few options that seem much more parsimonious - 1) that there actually is something to religion, and all these myths are attempts to explain some mysterious force in the universe (think the fable of the blind men and the elephant), 2) there's something about human nature that makes people keep inventing religion, 3) religion was present in the common ancestral group of all modern humans and has been passed down, being modified along the way (similar to language).
Another interesting observation is that, not just do many cultures have religion, but the deities in these separate religions serve similar, specific functions. For example, the Greek goddess, Ganymede, the Norse god, Thor, the Aztec god, Tlaloc, and the Egyptian god, Tefnut, are all gods of rain. The cultures all believed that rain was explained by the existence of these gods. It's a similar case in other cultures, and for many different phenomena. This would seem to indicate that the first of the three options above isn't true. It's not some mysterious force that early religions described - it was unexplained natural phenomena.
It seems probable that the reason all cultures have religion is down to the latter two reasons from above, and quite probably a combination of the two. Curiosity is part of human nature, as is a tendency to imagine agency where there is none. This could have very easily led early humans to conjecture that supernatural forces were controlling aspects of the universe that they couldn't yet explain. As people spread across the world and religious traditions were passed down from generation to generation, the slight differences in isolated groups led to the various religions that we have today.
Judaism (and hence Christianity), being monotheistic, may seem a bit different than the polytheistic religions. However, it appears that early Judaism was henotheistic (they believed in multiple gods, but worshipped only one). The earliest roots of Judaism appear to be from a prior Canaanite religion, with an entire pantheon of gods. Certainly, some passages in the Old Testament hint at these polytheistic origins (the use of 'we' in Genesis, the worship of other gods, etc.) And certain stories from Judaism are certainly from earlier cultures, such as Noah's flood being a rehashed version of the Mesopotamian Flood Myth. Many aspects of the Old Testament also read as just so stories explaining why the universe is the way it is - going back to what I mentioned above about religion providing explanations for unexplained natural phenomena.
It certainly seems that religion was invented by our ancient ancestors, not as a deliberate fabrication, but like I said, as an attempt to explain nature. The very concept of gods is part of that invention. That is very important, so let me repeat it - the very concept of gods is an invention of human mythology.
So, there are two key points from what I wrote above - there is no evidence for any gods, and the very concept of gods is a human invention. Given those two points, it seems almost certain that gods don't exist. It's a bit like unicorns, leprechauns, or fairies. Gods are just another set of mythical beings. We don't go around saying that gnomes are a remote possibility simply lacking in evidence - we rightly say that gnomes are products of our imaginations and never really existed. Why, when it comes to a different invention of human mythology, do so many people say that it's something we can never know for certain, or that deities are outside the realm of investigation, or that it takes faith to think they aren't real? These are things I would have said myself when I was still Christian, but now I recognize them for the double standard they are.