Could one ask an atheist, "What would proof look like if you had it?" Anything that you used as proof could be doubted, couldn't it, if you were really bound and determined not to believe. Water into wine? A good magic trick. Walking on water? He knows where the stones are. Loaves and fishes? The catering truck is parked where you can't see it. Resurrection from the dead? A hallucination, or maybe we only thought he was dead. Maybe the stars in the sky should spell out "TURN OR BURN!! ATHEISTS REPENT BEFORE I COME BACK AND IT'S TOO LATE."
Thanks once again, Keith Parsons, for that one.