In an effort to get God out of the Laplacean problem of being merely a quasi-scientific hypothesis which may be accepted or rejected on the same sorts of grounds as other theoretical entities such as phlogiston or quintessence, some recent theologians and theologically minded philosophers have suggested that God get out of the business of being. God, they say, is beyond being. It is a simplification but not an inaccuracy to say that religious folks who make God out to be 'beyond being' are in a direct line of ideological descent from some of the oldest heirs of Plato's Academy, and that the Platonists' One is the anti-onto-theologians' God.
Now why in the world would anyone think that the One is beyond being, or, which is what they really mean, that the One is not a being?
Actually what they have to say for themselves makes a lot of sense. Here's the gist:
Suppose we say that the One is a being. Then it is just like all other beings just insofar as it is a being. So there must be something besides being which distinguishes it from all those other beings. Well, it's the One, so maybe we could say, supposing it's a being, that what it has is the property of unity, or maybe the property of the tightest unity any being can have. No other being can have just this unity that the One has.
But the problem here is that we've had to divide up the One: it is a being with the property of being unified. There is its being and there is its unity. Those are two. So the One is not one. But this is crazy.
So these Platonists say that the One is beyond being--no division of being and modes of being, no division of subject and property. Just One. So their view has some intellectual heft.
However, it remains that their view is crazy.
What they should have concluded is that there is no such thing as the One as they were conceiving it. Their metaphysics had a fatal flaw which should have inspired them to rethink the whole enterprise. But they didn't, so the One went on muddying philosophy and theology for centuries.
Now the problem is not that they thought there must be a first principle of things. That's a very good philosophical and religious impulse which they acted on faithfully if erroneously. I'm all for first principles, in metaphysics no less than logic and ethics.
The problem instead is what they identified as the first principle, this One beyond being. They should have seen that that could not be the first principle. And in some sense they did see it, positing other secondary first principles to do the ontological work which the One obviously could not do: the Indefinite Dyad, the World Soul, the Logos, etc.
No, what they should have done is just said, yah, totes, the first principle is a being. Better, the first principle is being, and anything which comes from the first principle is a being because it comes from the first principle. It is the fundamental reality from which all other reality springs.
Once we've made that move, then we can just pack in as many properties of Being as we need, in order to have a system in which this first principle really can be the first principle. Logic? Number? Quality? Check, check, check. It contains all these. All the ways beings can be. It gives only what it has, and what it has is everything.
Since power to produce things other than the producer is one of the ways of being (which we know a posteriori), we can infer that the first principle has this power. So the first principle contains all things and has power to produce other beings, having the properties which it itself contains eternally. God makes little copies of himself, the Rock of our salvation scattered like sand across the cosmos.
This First Principle is the One-est One there can be. It is not the One of the Platonists, thank God, because their One is absurd. Turns out, the One-est One is a multiplicity in unity. Their are no true points, no true atoms, no being is an island, etc. The one includes the many.
If this it to work out, then what we need is some principled reason to think that being itself entails all the ways of being. This would get us close to divine simplicity as Christianity can in good conscience get.
You say, what about non-beings? If they are up to snuff, they too have reality. The snuff to which they must be up is, minimally, coherence (no square circles, even in God), maybe also goodness (no wholly cruel worlds, even in God's imagination). These non-beings are not truly not beings. They're tucked away in God, loved by God as his dreams, which we do not know is better or worse than being loved by God, as you and I are, as things he casts away from himself, babies from the womb turning back to God for nourishment and for a model of how to grow up.