I quite deliberately used the indefinite article "An" in the title for I in no way intend to speak for all atheists. As I've mentioned before on this site, I was raised a Christian, and that has no doubt influenced my worldview. If I were to create a creed for all atheists, it'd simply be, "I do not believe that gods exist." That is what is sometimes referred to as weak atheism (no insult intended). It is the type of atheism that animals and babies have, and that some adults claim to have (I'm skeptical about that). Strong atheism is the belief that there are no gods. Some people have a hard time distinguishing between those two statements, but the former is a lack of belief in gods, and the latter is a belief in the lack of gods. For those who use this terminology (and it's definitely not universal), there's also weak agnosticism – an uncertainty in one's beliefs, and strong agnosticism – a belief that such knowledge is ultimately unknowable for anyone. Again, the terms weak and strong should not be considered judgments about those positions.