Religious experience is just brain chemistry.
The objection, in full
Stimulate the temporal lobe, get a mystical experience. Take psilocybin, meet God. Epileptic seizures produce conversions; brain tumors produce visions of angels. If we can reliably trigger encounters with the sacred by poking neurons, the encounters tell us about the neurons, not the sacred.
Mystical experience tells us about the brain, just as a dream tells us about the dreamer.
or, in plain terms —If a brain scan can show why someone "feels" God, isn't God just in their head?
Mechanism is not refutation.
When I see a tree, light strikes my retina, neurons fire, the visual cortex assembles an image. I can describe every step of this process and the tree does not vanish. The mechanism by which I perceive is not an argument against the thing perceived. To say "your experience of the tree is just neurons" is to confuse the channel with the broadcast.
Religious experience has a neurological substrate. Of course it does. So does mathematical insight, romantic love, and the recognition of your mother's face. Showing the substrate has never disproved the object in any other domain. It is curious that this particular argument is deployed only against the sacred.
The mystic does not claim that her experience of God bypasses the brain. She claims that the brain, when properly disposed, perceives what is actually there. The atheist owes us a reason to think this case is different from every other case of perception.