 And if you are not a prophet, then you're a liar. Get away from you insignificant man. So basically they're giving him impossible challenges, showing their own hypocrisy, displaying that they will stay non-believers no matter what. It is very similar to the atheists nowadays, when they claim that they would need definitive proof of God. Even if God would appear right in front of them, it could be an alien species. Or they can't know if it then really is God or not.