If the sky breaks open, choirs of angels break forth, a 10km-long arm reaches down from the skies and an 8km golden-haired, bearded face looks down upon humanity and utters words of unshakable truth...then God is proven.
Well, although I believe some sort of supernatural entity created the universe, the scenario you describe would not "prove" the existence of God to me.
"What does God need with a starship?" James T. Kirk famously asked in Star Trek V.
What does God need with a beard, or a cohort of angels, or an arm or a face, or anything resembling the body of an earthly living being, for that matter?
If I see a 10 km-long arm in the sky, I will think "damn, that's an impressive hoax."