Science was once a child of the church.

[Cinjin]

I disagree, but I would like to hear what evidence you have for this claim.

You are the evidence; explain to me why Atheism would exist, if Theism did not?

Well, first of all, I'm a deist, not an atheist.
Secondly -
That's an odd response but hey, you're new here so I'll answer the question that I JUST asked of you.

The reason why atheism can easily exist without theism is because the default setting of all humans is lack of knowledge.
When you are born, you believe in nothing until some gullible halfwit attempts to imprint you with his own ideas of who created what.

However, not every person ever born on this planet has received such stupidity. There were and are still many who have never believed in a god - thus atheism. Now, if you're merely referencing the fact that the word theism is a part of atheism, than you're just arguing semantics and I have no time for such futile idiocy. I'm hoping that you're not just arguing semantics right?

Summation: The lack of knowledge of any god creates a person who does not believe in god. Therefore, atheism is completely possible to exist without theism.