I thought of a pretty cool argument a while back, and I figured that y'all may like it. Obviously, I'm going to be arguing from Platonist Idealism.
If someone creates something, the logical sequence (even if there's no temporal sequence) is basically this:
Concept of the thing in the mind (Knowledge) -> Intention to create -> The act of creation
For example, take a carpenter. Before a carpenter makes a house, he first has to know what a house is. Knowing what a house is, he has to will to make one. Willing, he then creates it.
Likewise in God we find the same logical sequence. God knows all possibilities in knowing Himself. Knowing all possibilities in knowing Himself, He wills. Willing, He creates from all eternity.
Clear so far?
What is obvious from the above is that both knowledge and intentionality require a proper object. You must know a thing. You must intend that thing.
Going back to the idea of "the impossible." When I say "the impossible" I mean contradictions. I say that God cannot make it both rain and not rain at the same time in the same place under the same circumstances...and so forth and so on. Yet, there are those who say otherwise. To them I answer in this fashion:
Suppose for a moment that "the impossible" can be willed. Well...ok. Well what's being willed? "The impossible" is not. It is not an object. It is
nothing. Ok. Therefore, if God wills the impossible, He must know, will, and create nothing.
Here's the problem though:
Summa Theologica, First Part, Question 14 wrote:
God is the proper object (and the only object) of His intellect. If God knows nothing, then since God can only know Himself, God must be nothing.
Said another way: if God wills the impossible, then there is no God.
The moment you posit that God can will the impossible is the moment that you slip away into atheism.