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

[Angrboda]

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.

If all you're saying is that God is existence itself, all you've done is declare a synonym for existence.  We already have words that denote being itself and declaring God to be synonymous with being itself doesn't add anything to the equation.  However if you are suggesting that God has an existence that is more than being itself, that He has consciousness for example, then your claim isn't covered by your predicate.  The existence or non-existence of a consciousness associated with being itself isn't falsified by that declaration.  You've simply equivocated on the meaning of "being itself."

2) No, I am saying that if something exists, then existence itself must exist. If nothing exists, then existence itself doesn't exist.

The presence or absence of things existing doesn't get you to a god that is being itself with a consciousness.  What you've uttered is what is known as a 'deepity'.  Taken one way, it's true but trivial.  Taken another way, it's extraordinary, but also false.  In attempting to capture a predicate that is profoundly true, you've failed by equivocating on the meaning of the term 'God'.

3) God's necessity isn't an assumption. The reality of "necessary being" is a conclusion drawn from the datum of existence. You can disagree with the conclusion, but that doesn't mean my conclusion is an assumption. The existence of being-itself is a conclusion drawn from the things we directly observe to exist. Being-itself, on the same logic that leads to that conclusion, is false only if nothing at all exists. It doesn't seem that controversial.

Bollocks.  God's necessity is a thoroughly ad hoc postulation.  Regardless, you don't believe that the existence of being-itself is the same thing as God existing, so your point is moot.