One of the current fads in analytic philosophy of religion (for analytic philosophy of religion goes through fad after fad) is 'modal collapse arguments'. People are always asking me about modal collapse arguments, usually with respect to divine simplicity. Like the fad about 'divine hiddenness' a while back, I don't really see much of value in these objections, although I suppose this argument at least touches obliquely on genuinely interesting modal issues. The problem is aggravated in that people regularly make assumptions in translating divine simplicity and the like into the modal framework they are using, without properly examining whether they are any good.
One version of the argument, if we slightly simplify that of R. T. Mullins and Shannon Eugene Byrd ("Divine Simplicity and Modal Collapse: A Persistent Problem"), is:
1. God's existence is absolutely necessary.
2. Anything identical to God's existence is absolutely necessary.
3. All of God's intentional actions are identical to each other so as to be one intentional act.
4. God's one intentional act is identical to God's existence.
5. Therefore God's one intentional act is absolutely necessary.
6. God's intentional act to create the universe is identical to God's one intentional act.
7. Therefore, God's intentional act to create the universe is absolutely necessary.
8. Therefore the existence of the universe is absolutely necessary.
What can be said of this argument? To be more exact, what can be said of it if I set aside my usual (but still correct) beef that translating noncomposition into identity in the usual sense of 'identity' is wrong?
It trades heavily on the assumption that anything identical to something necessary is necessary. This is not actually true in every kind of case. It has always been recognized that things can sometimes be contingently identical, and if A is contingently identical to something necessary, this would not automatically yield the conclusion that A is something necessary, simpliciter. To remove contingent identity as an option, you have to assume that the things being identified are rigid designators, i.e., that they exist in every relevant possible situation. (There's actually some dispute as to whether even this is enough, but we don't need to go into that.) This causes a bit of perplexity for the move from (6) to (7), because (6) refers to something that only exists in some possible situations. Are we taking it to be obvious that God intentionally acts to create the universe even if no universe is created? It's not clear why we would assume that 'God's intentional act to create the universe' refers to something that can be found even when we are looking specifically at the possible situation in which God creates no universe. If God's intentional act to create the universe is identical to God's one intentional act only when we are looking at some possibilities and not others, then it is not, in fact, absolutely necessary even if God's one intentional act is. But we can say the same thing for identity all the way through; if contingent identity is on the table, there is simply no reason to accept (2), which is a rejection of contingent identity. But it's unclear from (6) why we would not in fact consider contingent identity. Mullins & Byrd attempt to motivate (2) by the identity of indiscernibles, but this is illegitimate here; the whole force of the argument is that God creating the universe and God not creating the universe are not indiscernible -- they are discernibly different possibilities. While Mullins & Byrd also try to insist that you can't coherently hold that something contingent is identical to something necessary, this is false; the contingent can be contingently identical even to something necessary. For instance, the number of fingers I am holding up can be contingently identical to the number two, despite the fact that I and my fingers and my acts of holding different numbers of them up are all contingent but the number two is not. If you said otherwise, we could never count contingent things.
There are other problems with this. (3) is false in almost every actual account of divine simplicity. The reason is that an intentional act by its nature refers always to two things, the agent and the object. If I just say 'God exists', that refers to God. But if I say something like 'God knows Fido', then I am actually referring to God and Fido. Thus while God is the same in both cases, 'God knows Fido' and 'God knows Fifi' are not equivalent; and likewise, God's knowing of Fido and God's knowing of Fifi are not going to be the same no matter how much God is the same in knowing both, because Fido and Fifi are not the same. In doctrines of divine simplicity the argument is that this distinction is not a distinction between components in God, not that there is no distinction between them at all -- they are obviously distinct as to objects. And this is particularly relevant in that the intentional act introduced in (6) explicitly identifies an object, the existence of the universe. But the whole point of the argument is that the universe may exist or not. Therefore 'God's creating the universe' is a description that refers to two things, one that is, by (1), absolutely necessary, and one, the universe, that is not. Therefore 'God's creating the universe' refers to something that 'God's existing' or, for that matter, 'God's intentionally acting' does not, and they are not intersubstitutable descriptions.
And, of course, as previously noted, this is all even if we set aside the fact that the argument gets the doctrine of divine simplicity wrong by confusing noncomposition with what we usually call identity.