Proof is a fickle thing. What is counted as proof today can be dismissed once it is understood. The human heart is so defiant that every generation will find new ways to challenge the existence of God. Whatever proof is offered at any time in history, we will always demand something else. Remember, at one point in time, the world could not be explained. It’s existence was proof that God existed. Once we understand a thing, we lose the wonder we held for it.
I am sure that if God were to come down and send us a sign of his existence that the world would once again explain it away once they understood it.