Michael Shermer, at Skepticblog, writes:
Of course, no one is agnostic behaviorally. When we act in the world, we act as if there is a God or as if there is no God, so by default we must make a choice, if not intellectually then at least behaviorally. To this extent, I assume that there is no God and I live my life accordingly, which makes me an atheist. In other words, agnosticism is an intellectual position, a statement about the existence or nonexistence of the deity and our ability to know it with certainty, whereas atheism is a behavioral position, a statement about what assumptions we make about the world in which we behave.
When I left Christianity, I still considered myself a believer for a long time. I believed in some sort of god; I knew I didn’t believe in a Christian god or any sort of personal god, but I believed in the same vague, indescribable Something that so many seem to believe in. Then a day came when I realized I didn’t act like I believed. I realized I didn’t pray to God, and I didn’t ask God to guide my decisions, and I didn’t even think about God all that much anymore.
When we act in the world, we act as if there is a God or as if there is no God,
The day I realized that was the day I realized I no longer truly believed in God. I was acting as if there was no God, despite what I thought I believed. And once I realized I acted as an atheist, I became an atheist.
Unfortunately, a lot of people are still confused by the word atheist. They think that when I call myself an atheist, I’m in some way asserting that I know that God doesn’t exist. It’s not like that. Here’s my position. First, I do not believe in any sort of personal God, including the Judeo-Christian conception. I don’t believe in the Father, I don’t believe in the Son, and I don’t believe in the Holy Spirit.
Second, I do not believe it’s possible to disprove the existence of God, or, to put it another way, I believe it’s impossible to prove the nonexistence of God. You can’t prove a negative. God might exist, but then again, so might Zeus and the rest of the Greek pantheon. Which leads to point three …
Third, I believe the burden of proof is on those who make claims. I don’t claim that God doesn’t exist, therefore it’s not up to me to prove he doesn’t. Those who claim God does exist and that he manifests himself in the world — the burden of proof is on them to demonstrate this is true. Thus far, no one’s presented compelling evidence to make me believe it. And since the God question has existed in some form for thousands of years, without anyone presenting demonstrable proof of its claims, I have a strong suspicion it’s unprovable. If anyone could have proved it by now, someone would have. I think that if God does exist, we can never prove it.
So in sum, I think God’s existence is an unprovable question, which makes me an agnostic. But I also personally don’t believe in God, which makes me an atheist. There’s a term for this, and it won’t surprise you: agnostic atheist.