Proudhon

Thus, without a God or master-builder, the universe and man would not exist: such is the social profession of faith. But also without man God would not be thought, or -- to clear the interval -- God would be nothing. If humanity needs an author, God and the gods equally need a revealer; theogony, the history of heaven, hell, and their inhabitants, -- those dreams of the human mind, -- is the counterpart of the universe, which certain philosophers have called in return the dream of God.
And how magnificent this theological creation, the work of society! The creation of the demiourgos was obliterated; what we call the Omnipotent was conquered; and for centuries the enchanted imagination of mortals was turned away from the spectacle of Nature by the contemplation of Olympian marvels.

I consider myself a somewhat "angry athiest" but I really do not like this guy's reasoning process.

Proudhon. Was it?

It's true that our thoughts/mind greatly affect our perception of the world. But there's such thing as the concrete/fundamental truth. This can be rather obscure and foggy especially when contemplating over the abstract/vague concept. But that does not make the truth non-existent. To deny the truth is to deny the existence of your own self. Now you may want to claim that you don't exist or have never existed. But Triangulation research(I hope you respect science) indicates that you do have existed.

Now, God does not exist simply because he doesn't.

It's true that God was created in the minds of people. But that doesn't mean he will "disappear" when the humanity ceases to exist. He's fundamentally nothing, non-existant. Something non-existant can not appear nor disappear.