This will be true until a so-called god physically manifests himself and allows us to scientifically document his abilities. But even if this were to happen, under the guidelines of the Greater Miracle thread, I'd probably have to assume mass hallucination instead of there actually being a deity
All said deity would need to do would be to perform an action in plain view with objective, measurable effects. Something like diverting a river permanently would do, or even something simple like moving a boulder or carving words into a rock - or better yet, appearing directly on recorded media like a camcorder. Something that we can directly observe after the manifestation is over to show that the experience was not a mass hallucination.
This still leaves the possibility of a massive hoax, but depending on the miraculousness of the actions, that too can potentially be excluded.