What? No God?

If one says there is no God, it means that there must have been one? Atheists speak constantly and knowlegably about God and deny God. On the one hand, a bystander thinks, if the atheist is so adamant that there is no God, why then spend so much time saying no to the idea? If God doesn't exist, why worry about it? Why bother even denying what is not there? On the other hand, Atheism itself is a religion. It gives reasons for not believing, mostly second hand, and in so doing, attempts to find converts. It takes a lot of research because to say that something does not exist, you must first know what exists about it. It would follow that there must be some substance to the former in order to deny the latter. If there is substance, there is life, is there not? It is all a mystery to me this saying something is not there if it is there to say no to.