Atheists, tell me, a Roman Catholic: why should I become an atheist?

[Whateverist]

So you've just married existence with God, by definition. [1] So all you're really saying is that if something exists, something exists. [2] If nothing exists, nothing exists. That's a tautology, and you've added nothing to it by assuming God to be necessary and interchangable with existence. [3]

To be useful, it needs to be falsifiable in a way we can somehow test. [4] I can't run a test to see if stuff exists.  Obviously, it does, if I'm even here to run the test. So it can't be falsified. [5]

1) Right. That is what we say. God is existence, itself. God is his own existence, etc.

But why not just say that existence is existence itself?  Then God would just be God Himself.  If you say God is existence itself you need to say a whole lot more about why you say that.  Why does existence require this additional nom?  For that matter, what exactly does following it with "itself" supposed to add?  Is this like platonic ideals, that in addition to particular beings we must have somewhere the actual essence of which every actual being is a mere facsimile?  That didn't seem very tempting when Plato said it.  And doesn't do anything more for me in this context.