Atheists do not believe that God exists. If you find an atheist that says "I believe with certainty that there are no gods" I give you permission to punch them in their face.
Aaaaaaaand, you lost right there.
I have never seen anyone make a more contradictory statement.
Asserting something is true, or asserting something isn't true, it's still asserting something. If you said you don't believe in gravity, no one would be shifting the burden of proof if they told you to prove why gravity doesn't exist, because you just asserted that gravity doesn't exist.
You can't get out of "asserting" simply by making a negative "assertion". The only difference is that it's negative.