This response almost always occurs after someone says there’s no evidence of God. The fact is that no one is ever called upon to prove a negative, because it’s logically impossible. That’s right, it’s impossible to prove there’s no God. But don’t get excited yet. It’s logically impossible to prove the non-existence of any number of things. Think of anything we can’t directly observe. It’s logically impossible to prove that that thing doesn’t exist. This leaves the argument “you don’t know there’s not a God” on very shaky ground, since the same statement can be made for many different things, even things that contradict one another.
Let’s examine a claim that we can neither prove nor disprove: I am an alien in human form. If I told you this, you would not actually believe it no matter how hard I insisted it was true. My word would never be enough. You would require some sort of positive evidence for such an extraordinary claim. Needless to say, you wouldn’t be agnostic about it. You wouldn’t say “I neither believe nor disbelieve that you are an alien from another planet.” You would simply dismiss my claim as unverifiable, thinking I was pulling your leg, unless I came forth with very strong evidence (like building a homemade death ray).
“You don’t know there’s not a God.” This appeal to ignorance is very common, suggesting that the atheist should at least be agnostic about God. Surely if they were consistent, they would become agnostic themselves? But we don’t have to be agnostic. One of the best indicators that something isn’t there is a lack of evidence for its existence. Religion has never made any discoveries or predictions about the world, so it must have nothing to contribute as to its true nature.