Absence of evidence is not evidence of absence: always? Maybe not.
I think that, sometimes, the lack of evidence in favour of a positive assertion does constitute evidence against that assertion. For example, the Noahic flood cannot have happened as described in Genesis because, if it had, there would be clear evidence of it. There is none – and there has been quite a lot of investigation into the available evidence.
We can do the same regarding claims of the existence of God, depending on the god-concept being put forward. If the claim is for a God that intervenes in the world in such a way that those interventions can be detected, then there should be good evidence of such interventions (there is none).
On the other hand, if God (or God’s activity) is undetectable, then that is the functional equivalent of there being no God.
In addition, if God exists but God (or God’s activity) is undetectable, then one must wonder on what basis humans first began to think that God exists. Might that not indicate that God is a product of the human imagination? Believers might make the ad hoc claim that God used to be detectable (“back in the Bible days”) but has since decided to become undetectable. They must acknowledge, though, that it seems a bit suspicious that God chose to withdraw just when humans started getting pretty good at scientific investigation beyond what is readily available to unaided human senses.
Some will claim that God chooses not to be available for inspection because that would violate our “free will”. According to the Bible, God used to frequently put himself on display. Why was he not concerned with “free will” in those times?
What about the Devil? If he existed, he would certainly know that God exists. If the Devil had “free will”, then he would have sure knowledge of God’s existence and yet he would have chosen to oppose God. If the Devil did not have free will, then we would have to conclude that God had chosen to introduce Evil into the universe (therefore, God would not be wholly good), unless we were to think that the Devil has always existed alongside God, which raises the Devil’s profile considerably. (That might appeal to people who hold certain mystical/theological views but I expect that it wouldn’t fit neatly into mainstream Christianity.)
But I’m getting off track. I think it’s pretty clear the the “free will” justification for God being undetectable doesn’t hold up well even under the rather light scrutiny it has received in this blog post. So, unless you’ve got an even better way to justify God’s being undetectable or, more boldly, unless if you’ve got some testable evidence for God, then I’m going to suggest that, even when atheists make the positive assertion that God does not exist, the burden of proof may still fall upon the theist’s shoulders.