An excellent post from Atheist Revolution:
"Why are you an atheist?" I've heard that question or some version of it more times than I can count. I don't blame the person asking it for asking it. Like me, they might live in a country where almost everyone believes in gods. Someone who doesn't is an outlier. Someone who uses the "atheist" label is even more of an outlier. Besides, asking questions about the world around us is a good thing. It is the kind of thing we should encourage.
My most common response is that religious believers haven't met their burden of proof. I'm an atheist because I have not encountered sufficient evidence to support a rational belief in gods. But while I find this response satisfying, I realize that few religious believers will do so. I also recognize that this response is incomplete. I've sacrificed clarity for brevity.
What's an Atheist?
No atheist should attempt to answer this question without doing something first. What's that? Make sure we understand what the person asking the question means by "atheist." Chances are, they mean something different from what we mean.
When I say I'm an atheist, I'm saying I don't believe in gods. That's all I'm saying. I'm not claiming to know that gods don't exist. I'm not claiming to know anything about gods with certainty. I'm telling you I don't believe in gods.
Why is this important? If the person asking the question thinks that an atheist is someone who is 100% certain gods don't exist, we're going to talk past each other. They're going to ask me why I believe things I don't believe.
It is also important because it gets to why I'm an atheist: evidence. I don't have enough of it to conclude that gods exist. Until I do, I'm operating "as if" they don't.
Why Is Evidence Valuable?
I'd like to believe as many true things as possible. I'd also like to believe as few false things as possible. I won't pretend everyone shares this desire, but it applies to me. How do I go about doing this? That's where evidence comes in. I haven't found a more reliable way to do it than evidence.
When someone makes a claim, we seek evidence to help us assess it. The evidence we seek usually scales to the claim. Suppose the claim is mundane, ordinary, or trivial. We may not need much evidence to support it. When the claim becomes fantastic, implausible, or extraordinary, we'll demand more. Most of us operate this way for most claims.
If I text my co-worker right now and tell him I had a haircut, he's not going to need any evidence to support my claim. But what if I tell him I had a haircut and I now have a mohawk? He's not going to believe me without evidence. He's going to demand a photo.
What does this have to do with the question of whether gods exist? Plenty. Consider the claims you often hear from religious believers about their gods. If you hadn't grown up hearing them, how plausible would they sound to you? Are these not among the most extraordinary claims we've heard? Why don't we need considerable evidence to support them?
What Evidence Do You Need?
For some religious believers, evidence is irrelevant. Some think the existence of their preferred god(s) is self-evident. How else can anybody explain why we're here? Others value faith, which they recognize as belief without evidence. They think faith is virtuous and worry that evidence could render it unnecessary.
Many other religious believers will point to specific evidence. As far as I'm concerned, the most valuable type involves personal revelation. If your preferred god(s) have been communicating with you, that seems like evidence. What if every one of your prayers has been answered? Why wouldn't you believe?
But as valuable as personal revelation might be for the person who has it, it holds little value for the rest of us. I've never heard from gods or felt their presence. Years of prayer didn't matter. If gods are out there, they've never made themselves accessible to me. It would make no sense for me to believe. Hearing about someone else's revelation doesn't cut it.
It would be easy to get bogged down in the question of what sort of evidence I'd consider sufficient. The short version is that I'd need my own personal revelation. I'd need one of these powerful gods to make itself known to me in a way I couldn't mistake for anything else. I'd leave the specifics up to the god(s) involved. If such a being exists, I'd have to think it could show me if it wanted to do so.
Of the Possibilities, Which Sounds More Likely?
Despite our best efforts to distance ourselves from it, nature surrounds us. We are comfortable with natural explanations. Few of us believe that the raindrops landing on our windshield are the tears of gods. We may not enjoy the sound of thunder when we are trying to sleep, but we know Thor isn't behind it.
While there are plenty of things we can't explain, supernatural entities aren't needed. They don't have any more explanatory power than saying, "We don't know." We know that nature exists. We live in a natural world. When we weigh that against gods, which seems more likely?
If there's an all-powerful being out there pulling the strings, why does it so rarely show up where we most need it? Did it get you that sweet parking space while watching your neighbor's kid die of leukemia? Why do the people who claim to speak for it often seem so despicable?
Take a look an honest look at our world. Doesn't it resemble the sort of world we'd expect if there weren't any gods? Why do you suppose that is? It could be because the gods are hiding. It also could be because they aren't there. But which seems more likely?
Remember, I'm Answering Your Question
Some religious believers become defensive or even hostile in conversations like this. They accuse the atheist of trying to change their minds or even destroy their faith. This is the point where it helps to remember how we started down this road. You asked a question, and I'm giving you my answer.
I didn't knock on your door when you were busy. I didn't greet you by asking, "Have you heard the good news about atheism?" I didn't drop flyers on your porch promoting an atheist organization after you told me you weren't interested.
I didn't do any of these things because I'm not trying to change your mind. I don't know what you've experienced. I'm not sure which gods might have communicated with you. What I do know is that you asked me why I'm an atheist. I'm attempting to give you an answer.
He might consider coming to a mutual definition of the word god with the person asking the question. Generally speaking I don't believe in the same god(s) he doesn't believe in.