Let's suppose for a moment that God did do what many atheists here have said He has not done, and reveal Himself to mankind in a way that is undeniable.
How would you know it was God revealing Himself and not just some natural event with a natural explanation?
Let's suppose He caused a message to appear in the clouds in the sky over Israel. How would you know it was really God doing it and not just a chance, random arrangement of clouds to look like words?
Or let's suppose God appeared to you, an atheist, personally. How would you know it was really God and that you were not just hallucinating?
Let's suppose He appeared to hundreds of people at one time and did miracles. How would you know that it was not just some magician doing magic tricks with the help of people who were working secretly with the magician?
Let's suppose a booming voice from the sky cried: "I am God and Jesus Christ is my beloved Son, listen to Him!" How would you know it was not just you imagining the voice inside your head?
I keep hearing over and over again: "If God is real, then He can make His presence known." Well heck, I agree!
But if He did, how would you distinguish the supernatural from the natural?
What marks or qualities would such a revelation possess? Would you even be able to recognize it?