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

[Ben Davis]

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

If it already has a label, why rename it 'God'? And how do you tie this nebulous description back to the very human descriptions of the Catholic God in the bible?

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

Then once again, why call that 'God'? Why not simply stop at 'existence'?

3) God's necessity isn't an assumption. The reality of "necessary being" is a conclusion drawn from the datum of existence.

Seriously? I would challenge you in the strongest terms to provide even the slightest evidence to that effect. I predict that you will be incapable.

You can disagree with the conclusion...

Thank you, I just have.

...but that doesn't mean my conclusion is an assumption.

Disagreement doesn't however the fact that there can be no evidence to support such a conclusion does. There is no evidence to support your conclusion therefore it is an assumption at best.

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.

I know the bible says that but unfortunately, reality doesn't agree with it.

4) I dunno. The mere logical consistency seems to be enough. If I say that all cars have doors, that proposition is falsifiable in principle simply by the concept of a car that doesn't have doors. In principle, if you observed a car that did not have doors, you will have succeeded in demonstrating the proposition false. There is a difference in falsifiability (the principle abstract concept upon which a proposition is proved false) and the demonstration of falsity (the concrete evidence of the principle abstract concept which contradicts the proposition). 

Almost all correct except that you don't test for falsehood, you test for veracity.

5) It could be falsified in principle if nothing existed. It can't be falsified (in fact) because of the obvious fact that some things evidently exist. It is the easiest possible experiment.

Once again, you're nearly right nearly however in this case, defining falsifiability creates impossible tests: you would have to compare existence with non-existence. Since it's impossible to produce non-existence, claims reliant on such tests have to be dismissed as unfalsifiable. Instead, the requirement then becomes 'demonstrate existence' and this is something that all 'god' concepts have failed to do so-far, spectacularly.