You could do the whole "knowledge is a True Justified Belief" thing.
So did they "know" the earth is flat? They certaintly believed it and could justify it with science of the time, but it wasn't true.
If you guess a question on an exam and get it right, did you know it? No, you could say, because you didn't believe it was the right answer and it wasn't justified.
Can you justify a false belief? Certaintly, you see it every day.
So some argue you need to believe what you are saying is true, be able to justify why you believe it, but also have that justified belief correspond validly with the facts of the external world, in order to claim you "know" a certain thing.
Now, you could argue you can't "know" either way if God exists, due to the nature of God. It cannot be proven that he does or does not exist technically. So if you want you could say both sides have Justified Beliefs, but which side is more Justified? The one relying on subjective evidence or objective research? Now while the objective research hasn't proven God doesn't exist, and you could say no test ever could (how can you prove something otherworldly in nature doesn't exist? I cannot prove unicorns don't exist, I can just say nobody has proven it to exist) but I'd say the scientific framework of viewing the world and what exists has far more JTB's attached to it than the spiritual at this point.