Is atheism a belief?

[Deesse23]

If the condition is fulfilled, you believe, if not, then you dont. No?

That's right.  You can believe to the extant that    X is true, but have no way of knowing whether X is true.

What does that have to do with you (not) knowing what you actually believe?
The word "know" in this sentence has nothing to do with the content of your belief, just with your belief per se. In your quote "know" is used in relation to the content. This is not the same.