Oh right, I forgot, claiming a non-contingent object exists, and calling it God, is proof.
What proof do you have that the universe is contingent?
Namely, that that the universe
began to exist and therefore could have
failed to exist. The atheist wants to avoid at all costs the conclusion that the universe is a contingent object. So what's the atheist to do at this point? He can say that
yes, the universe does have an explanation for its existence and that is that the universe exists by a necessity of its own nature. Thus the universe could serve as a God-substitute that exists necessarily.
Yet there's a reason why most atheist philosophers are extremely hesitant to embrace this alternative. None of the things that make up the universe - stars, planets, galaxies, radiation, people, animals, etc - seem to exist necessarily. That is - they could fail to exist. Indeed, at some point in the past, when the universe was very dense, none of them did exist. The objection that typically follows is that the matter which these things are made of simply exists necessarily and all these things are merely different configurations of matter. Yet according to the standard model of subatomic physics, matter itself is composed of tiny fundamental particles that cannot be further broken down. The universe is just a collection of all these particles arranged in different ways. But now the question arises: Does each and every one of these particles exist necessarily? Yet it is immediately clear that these particles
don't exist necessarily because these particles aren't composed of anything - they just
are the basic units of matter.
It isn't necessary for the universe to exist (in other words, the statement "the universe
must exist" would need to be supported by the atheist. Yet there's a reason why no serious atheist philosopher would ever sincerely support this claim - the burden it places on the supporter is far too heavy to bear). We generally trust our modal intuitions on other familiar matters (for example, our sense that the planet Earth exists contingently, not necessarily, even though we have no experience of its non-existence). If we are to do otherwise with respect to the universe’s contingency, then the non-theist needs to provide some reason for his skepticism other than his desire to avoid theism.
What proof do you have that God is not contingent?
God, properly understood, cannot have begun to exist; God is a necessary being whose existence lies in the necessity of His own nature. It's impossible for God to have a cause (a more powerful God creating another God? An infinite regress of God-creating-Gods?).