Atheism says that the cosmos and everything in it arose by random chance. That's ridiculous!

Atheism does not say any such thing. Atheism is nothing more and nothing less than not saying "there is a God." It does not present views on cosmology, biogenesis, or anything else.

Attackers of atheism will often construct straw man arguments that begin with the words "Atheism says" or "Atheists say". Because atheism literally says nothing, these arguments can be dismissed out of hand.