If you’re not sure:

Where I come from, if you don’t believe in God and you don’t have a proof that God doesn’t exist, you say you’re agnostic. A typical conversation in polite company would go like this:

Woman: What are your religious views?

Me: Oh, I’m an atheist. You?

Woman: Well, do you know for certain that God doesn’t exist?

Me: I’m pretty sure, that’s what I believe.

Woman: How do you know that God isn’t withholding all evidence that he exists to test your faith? How do you know that’s not the case?

Me: Well, it’s possible that everything is an illusion.

Woman (with finality): You’re agnostic.

Every community has its own set of definitions. Here on LW, you are an atheist, simply, if you don’t believe in God. You don’t have to be 100% certain – we understand that nothing is 100% certain and you believe in God’s non-existence if you believe it with the same conviction that you believe other things, such as the Earth is orbiting around the sun. For a fuller explanation, see this comment.

For the rest of us:

My favorite passage in the Bible is Exodus 4 because this is the part of the bible that made me suspect that it was written by men; men that were pretty unsophisticated in their sense of justice and reasonable deity behavior. God asks Moses to come be on His side, and Moses agrees. The next thing that happens is that God is trying to kill Moses because his son isn’t circumcised. I guess God already asked Moses to do that? They left that part out of the story. Nevertheless, God seems more peevish than rational here. Presumably, he chose Moses for a reason. So trying to kill him in the very next scene doesn’t make a lot of sense.

As someone who has had some trouble figuring out how things are thought about in atheist circles, I would like to suggest not being like God in Exodus 4 when people ask why we’re atheist even though we can’t prove there’s no God. It’s understandably annoying to repeat yourself, but they need to understand the context of atheism here. You can refer them to this comment again or "The Fallacy of Gray" or here.

And steel yourself for the inevitable argument that belief in God is a special case and deserves extra certainty. These are final steps…

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I would like this to be a reference for people coming onto the site that consider themselves agnostic. Any editing suggestions welcome.