In the Wizard of Oz the witch simply wrote in the sky. No one had to doubt that she was there. They could see unmistakable proof! No one had to profess 'faith' or 'belief' in things invisible and unprovable.
If I woke in the morning and saw God's handwriting in the sky [and somehow knew it wasn't an elaborate hoax], or saw him rearrange the stars in clusters that stated "I am real, I am God." or something like that - then OK. I accept it and begin the process of 'knowing God'.
He won't do that - because he isn't there. It would be super simple -