How often have you been told that your atheism isn't a belief, rather it's a lack of belief? Even really smart people say this, as if it makes their position stronger than yours. It's never felt right to me. Even atheists say it sometimes. Ricky Gervais said something like, "Saying atheism is a belief system is like saying not going skiing is a hobby. I've never been skiing. It's my biggest hobby. I literally do it all the time." Although he has a way with words, I don't agree with Ricky on this one.
If you look up "atheism" in a dictionary, you will invariably find a definition something like, "lack of belief in the existence of God or gods." It sort of makes sense, but two things occur to me. First, that lack of belief is a conclusion, and no sensible person reaches a conclusion without basing it on a foundation of what they believe to be true. And secondly, I'm with Jean-Paul Sartre when he said, "If I do not choose, that is still a choice." In the same way, if I don't believe something exists, then I still have to believe that the non-existence of that thing is the truth. In other words, it's still a belief.
Let's ignore the philosophical bit and talk about reaching a conclusion. Here is an example. Some Christians believe the world to be about 6,000 years old. They believe this because Joseph's family tree can be traced back through New and Old Testament and it generates a 6,000 year timeline. Add another 2015 years to that and you have the age of the earth. In fact, you have the age of the universe because God created it all in the same week.
I personally believe the world to be much older. I believe this because there are many ways of dating things which contradict what it says in the Bible. We see light from distant stars that has taken far longer than 6,000 years to reach our planet. We have an unbroken dendrochronology record of British oak trees dating back more than 10,000 years. We can map the progress of the continents across the surface of the earth going back millions of years. We can radiocarbon date items far older than the earliest biblical times. All this points to an earth much older than 6,000 years. Thus, I conclude through my beliefs, that I don't believe the Bible, at least on this issue.
Looking at this from another angle, is Christianity simply a belief in the existence of Christ? I don't think it is. I'm certainly no Christian, but I don't discount the existence of Christ. In fact I'd suggest it was quite likely that someone called Jesus probably did walk the earth. Muslims believe in Christ. They wouldn't call themselves Christian either. I'd suggest that to be a Christian you have to believe that Christ was the son of God and that he died for our sins on the cross. As such, isn't Christianity also a collection of beliefs which form a foundation for a conclusion. That's not so different to atheism is it?
I think one has to think of atheism as a philosophy in the same way that religions are generally considered to be philosophies. There are no rules and certainly no set list of beliefs that make me an atheist. I do believe in evolution for instance, like most atheists, but it's not a prerequisite. Some Christians believe in evolution too. Conversely, some Christians believe in the virgin birth, but not all.
So, if, like me you are slightly insulted when some fool reduces your carefully thought-out philosophy to a "lack of belief," I think you should challenge it. Ask this person how strong his belief is. I don't know a single Christian who avoids shellfish for religious reasons, though the Bible says they should (Leviticus 11:9-12). I don't know anyone who avoids clipping the sides of their beard (Leviticus 19:27), or avoids tattoos because the Bible says they should (Leviticus 19:28). These are all examples of lack of belief aren't they? Who lacks more belief, the Christian or the atheist?