Voluntarists say just that: God can make contradictions true. And if someone is really prepared to say that contradictions might be true, it's not exactly clear -- to me, at least -- how to answer. But I'll confess that I've never understood the pull of this solution.
Here's a way of getting at what bothers me. Suppose, to see if it could make sense, that there's an omnipotent God. (Our goal is to see if the concept is coherent; not whether it fits any actual thing.) Suppose we have a computer screen with 1280 x 720 pixels. (Let them simply be on or off; ignore color.) Suppose we ask God to turn a set of pixels on so that there's a circle on the screen. (We have to allow for a certain amount of approximation, but that won't affect the real point here.) God can easily do that. (So can anyone with a good Paint program.)
Now suppose we ask God instead to arrange pixels so that there's an equilateral triangle on the screen. Once again, no difficulty. But now suppose we set God a third task: turn on a set of pixels so that the result is a figure that's both a circle and an equilateral triangle.
The problem, I suggest, is that nothing would count as success. There are 21280x720 combinations of "on" and "off" for the pixels. God can arrange the screen in any of those ways. But none of those ways count as making a circle that's an equilateral triangle. I suggest the reason that God can't do what we ask is that there's no coherent task to be done; nothing counts as doing it. What we've asked God to do is what I've elsewhere called a "pseudo-task," and the suggestion is that it's no limitation on God's power (nor any other being's) that s/he can't perform a pseudo-task.
Some people balk at this. They say that if God can't are contradictions true, or can't make triangular circles, or can't make 2+2=5, then God isn't "absolutely sovereign." Let's grant the term: a being that can't bring contradictions into being lacks "absolute sovereignty." But so what? Why is this of any religious interest at all? Why would a God who can't perform pseudo-tasks be unworthy of our worship?Why care about "absolute sovereignty" defined this way?
Think about the stone. You want a ten-ton one? God can make it. You want one that weighs as much as the rest of the world? God can do that. Pick your weight; God can make it. Why, if I'm a believer, should I be bothered by the fact that there's a certain weird, self-referential "task" with an extra step that God can't perform? After all: for any weight, God can make a stone that has it. And for any weight, God can lift a stone that heavy. Put those two bits together and it follows: God can lift any stone, no matter how heavy. How would God be even more powerful if there were a weight beyond his capacity to lift?
Perhaps there's something the voluntarist is looking for that matters, but I can't for the life of me see what it is. I think I understand what it means for there to be a "task" that nothing would count as performing. Do we need to say that God should be able to perform a task that nothing would count as performing? Why? What do we mean? What possible logical, theological or devotional reason could we have? We can allow that God is beyond our understanding. But that doesn't get us to the conclusion that God must be able to perform pseudo-tasks. Or if it does I've never been able to see why.