Arguing the ontology of god with a theist will almost always get them making the case for god's existence being necessary. That is to say, they will argue that god must exist, because it is logically impossible for god not to exist. To do this, usually some version of the ontological argument must be made. I don't want to address the OA here because that's not my goal. Rather, I want to instead address something I stumbled upon through debating theists over what I think is one of the toughest questions you can throw at any theist. It is a question I asked in a recent post I made, why I'm an atheist.
In the post I made the argument that the god of classical theism is not logically possible and to demonstrate that I asked, "How does a timeless god who knows everything freely chose to create our world and not some other world?" It could also be asked referring to the universe, "How does a timeless god who knows everything freely chose to create our universe and not some other universe?" Go ahead and ask any theist (or deist) this question and then pay close attention to their answer.
First, notice that the question is a "how" question and not a "why" question. One Christian who responded to this question said, “Uh….how would I know why God chose our universe vs another universe?” This response totally misses the point, for the question is merely concerned about how it could be possible for a timeless god to create one particular universe over another, and not why (although once the how question is attempted the why question becomes more relevant as we'll see).
Second, there are at least two major interpretations that philosophers of religion have on god's relationship to time. One is that god is intrinsically timeless with or with out an act of creation, the other is that god is timeless prior the creation of the universe, and temporal subsequently. Neither of these views of god's relationship to time make any difference here because the question is concerned with god's choice prior to creation of our universe. There are some who believe god is fully temporal, or at least omni-temporal (existing at all moments in time), and that god exists in some sort of metaphysical time prior to physical time, but I've argued that the idea of metaphysical time amounts to nothing more than philosophical wordplay, whereby the theist unjustifiably claims god has time without having time.
This question occurred to me when I was thinking about Einstein's hypothetical question, "Did God have a choice in creating the universe?" I even wrote a blog about it here. The answer I've come up with is "No." God could not have a choice in creating the universe because choices require time and states of indecision. If god is omniscient and knows everything, then he knew that he would create our universe, and not any other universe, and he knew he would create a universe and not refrain from creation. For god to have had a choice in creating the universe, he would have had to exist in some moment or in some mental state where he was unsure of whether or not he would create a universe. For this to be possible god would have to exist in time and there would have to be something god couldn't know, namely, whether or not he'd create a universe. Such a god could not be timeless and omniscient, as the god of classical theism is described to be. And thus, the god of classical theism is not logically coherent.
To get around this, some theists have said that god simply "exists" with the intention to create a universe eternally. William Lane Craig, for example recently said this in his debate with Lawrence Krauss last year. The problem with this idea, is that it is not enough for god to simply "exist" with the intention to create "a" universe, he'd have to exist with the intention to create "our" universe, otherwise god faces the same problem of needing time to make his decision on what universe to create and he'd also have to exist in a state of indecision prior to deciding what kind of universe he'd create. So now let's suppose that god exists timelessly with the intention to create our universe and not any other. Now the "why" question arises. Why does god exist timelessly with the intention to create our universe and not any other universe, or no universe? It doesn't seem possible if god exists timeless with the intention to create our universe that the creation of any other universe or no universe can happen. For if god's mind is timelessly locked into creating our universe, no other possibilities can occur. So how is the theist going to answer the why question? It seems to me that they must come to the idea that god's eternal state to create our universe is a brute fact, because it is logically impossible for god to have had a different intention if he exists timelessly with the intention to create our universe.
I haven't heard many theists give reasonable answers to this question. I hardly think one can make the case that god has to exist with the intention to create our universe out of logical necessity, and that it couldn't have been any other way. What justification could possibly make that argument? I don't think there is one. If the theist gets this far, they will also have admitted that our universe is not really contingent but exists necessarily because it is impossible for it to not have existed. Thus it seems that if the theist is with me this far, he or she might have to give up, claim ignorance and perhaps admit that god's intentions exist as a brute fact with no explanation. But I suspect that many theists will avoid coming to this conclusion as best they can and will focus on avoiding the logical outcome of the timeless, omniscient god at some point prior.
I suspect the most likely place theists will object to my conclusion is where I argue that a timeless god cannot make decisions because that requires time and states of indecision. They will want to argue that god somehow can do that which requires time, without time. But any event, whether mental or physical requires time, because there will always be a "before" and an "after" the event. And even if I let this slide, there is still the problem that god's omniscience poses to his ability to make decisions. If god somehow knows he will create our universe, there is no possibility he can create any other. It's like "knowing" you will turn left at the upcoming intersection and not right; turning right is never a possibility. So why does god exist timelessly with the knowledge he will create our universe and not any other? Is there a logically necessary answer, or is it "just because"? If there is a logically necessary answer, what is it? Why couldn't god exist with the intention to create no universe or one that is even just slightly different than ours? It seems the theist will at least have to consider that the answer might be a brute fact.
Most theists will probably admit that they cannot answer this question, both the "how" aspect and the "why" aspect, but they'll probably just insist that there's an answer somewhere out there and we just can't know it. In other words, it all makes sense, but we're just in an unfortunate epistemic disadvantage because we're finite human beings and god's ways are higher than ours. You can see how this answer is not going to satisfy many atheists. Saying, "there's an answer, we can't know it" is the ultimate cop out that can be employed anytime a logical inconsistency arises in one's worldview. It's just far too easy to make an assertion of this sort, and then absolve yourself of any explanatory responsibility. It is my position that there is no way to answer this question because it exposes how the god of classical theism is logically impossible, and given the other evidence we have in favor of naturalism over theism, there's no reason for me to believe - on faith - that there's some magical answer out there that we'll never have access to while we're alive.
The atheists says the universe (or multiverse) most likely just exists without explanation and the theist throws the principle of sufficient reason in his face as an argument against the existence of brute facts. But if my logic above is correct and so are its conclusions, then it seems that the theist must also claim that god's eternal intention (which could not have been different) to create our universe is also just an inexplicable brute fact. And saying that god "freely chose" to create the universe is indefensible.