Debunking Christianity? It's actually quite as simple as asking "why?"

[Minimalist]

Let's say you were an all-powerful deity--not necessarily the Christian God--and you had an interest in creating one of Plato's Forms: say apricotness. You couldn't really do such a thing without creating the ante-form, the lack of apricotness, could you?

Our resident fundies will denounce you for suggesting that their "omnipotent" god can't do something. If he can't create apricotness without also creating the lack of apricotness then he is not omnipotent. And they hate it when you say that their boy can't do whatever he wants.

I was taking for granted that God existed and that we were comfortable with that

First mistake. But welcome anyway.