Do you see the difference? One is a personal assertion of non belief, the other is an affirmative statement of belief that there is no God.
Many atheists define atheism as a simple lack of belief, and for many, I think that's accurate. For those, though, that go the step of saying that there is no God, that's a belief just as sure as any religious belief and some atheists will preach this gospel (that there is no God) like an evangelist with an aim to converting people to their belief. Sometimes they end up behaving just the same way as the believers they (maybe even rightfully) mock, resent and ridicule.
Just something I always thought interesting.
re·li·gion
1) the belief in and worship of a superhuman controlling power, especially a personal God or gods.
2) "ideas about the relationship between science and religion"
3) synonyms: faith, belief, worship, creed; More a particular system of faith and worship.
4) a pursuit or interest to which someone ascribes supreme importance.
I'm an atheist. If I'm asked if I believe in god. My answer is simple. "NO!" Then comes the question: "Well don't you think that there is at least a possibility that there is god who created the known universe?" Again, my answer is simple. "NO!" In other words I can clearly look someone in the eye and say, "there is no god" in my awareness or reality. Nothing more needs to be explained or said. God is not a part of my mental vocabulary. The word (or the meaning of the word) simply isn't relative to my life. I don't ponder the possibility. There is no element of, or any concept related to "god" that nudges my curiosity. I don't have the desire to consider it relevant to anything that involves my existence...or the existence of anything that I'm capable of being aware of.
A belief does not constitute a commitment - or to put in practice a personal effort in order to sustaining, enhance, or show allegiance to a said belief. To say, "There is no god" requires virtually no mental, emotional, or physical investment into making clear the intent and meaning of the statement or comment by the person who makes it.
When something is absent from one's physical, mental, sensual experiences...or otherwise. How can one sustain or build on something that doesn't exist to them?
Are there things that doesn't exist in my experiences that might, in fact, exist? I would have to assume that's more than possible. But I must confine my beliefs to those possible unknowns to also be empirical in some shape, form, or fashion. There's a lot of places in the universe I've never been to or experienced. Obviously, I've never been approached by a supernatural entity either. If I can't be inspired, compelled, have access to, or somehow be motivated by things unknown, that are out of the realm of my awareness. I'll automatically turn to those things derived by my own nature and experiences that I rely on to help me navigate through my life. God is not one of those things. God simply doesn't exist in my reality.