from the history of mankind, human already have a concept of a perfect being even before the holy scriptures were not been written. Did someone tell them? No they already have that knowledge from when they born. You can imply the ideology of rationalism here. So theist claims about God is the first one.which came first, the claim that god exist or atheism? your implying that atheism claimed no god exist as if independent to the god claim, if the god claim didn't exist, atheism wouldn't exist.
I'm sorry, let me rephrase it, when you say no god exist or dont believe that god exist thats a claim which carries the burden of prof.
The reason why some people are questioning about God existence are:
First when human suffered from many kinds of problems, and disasters. There is no God, if God exist life must be perfect.
second when science evidences discredited creationism. (well for atheist)
there is more reason why atheist exist. I stated those two because those are the most common.
Now what is the reason you became atheist? If your reason is in those two meaning you're standing in a belief that God didnt exist because of this and that. conclusion? You used that reason to "claim" that there is no God.