Personally, my mind would be blown. I’d have to radically change my understanding of the universe to comply with the fact that there has formerly existed a god in it. I’d first want to know what sort of god we’re talking about here, though, and if there’s any connections with the beings the religions made up.
I’d be on the lookout for radical changes in the universe, to figure out what exactly this god has been doing all this time, and what difference its death will make. Pending that, I wouldn’t know what to think of it yet.
The scientific community would explode into excitement, too. New fields would be born. Theo-biologists would want to do an autopsy to learn how its body worked, and if it has cells with DNA like us, and how it can be humanoid without fitting into the evolutionary family tree. Theo-neurologists would want to understand how its brain worked, and theo-physicists would want to know where this thing had been living all this time and how it accomplished what it would have done.
It would be our first look at an extra-terrestrial life form.
Even though I’m an atheist, it would be madness to expect my life wouldn’t change. It would be madness to expect world politics wouldn’t be turned upside down and inside out and tied pretzel-shaped knots into. It would be madness to expect this wouldn’t be a more important historical event than the world wars.
And imagine if what this god had been doing all this time was veering dangerous meteorites away from Earth, direction black holes safely around us, keeping blood-thirsty aliens from discovering us, all just outside the reach of our telescopes.