For the reasons just given, I think the main choice is between Naturalism and Monotheism (which I will sometimes call Theism for short).
Before trying to decide between these two views, I think it is well worth emphasizing just how much they have in common. Both of them agree that Nature is not a divine being, but instead a limited reality capable of being studied and explored—these metaphysical views are therefore the most compatible with Science, and it is not surprising that people of a scientific bent tend to adopt one of these two views.
We thus have to decide which conception to adopt of Ultimate Reality. Is it more like a Law, or more like a Mind?
On the one hand, there is something strange about the concept of a “Law of Nature” as a fundamental entity, since as pointed out earlier, normally a law is something which is imposed by a Lawgiver. Really it must be a metaphor for something stranger, more magical as St. Chesterton says. Indeed, in so far as the Laws compel matter to behave in a particular, rationally comprehensible way, one can see that they are already, in certain respects, a bit more like a living mind than say a rock is. Thus Naturalism itself borders on a form of Theism, to the extent that it implies that the Universe is governed by a rational ordering principle (λογος) similar to, but greater than, the rational ordering in the minds of the scientists who study it. There is a risk here of introducing at least Einstein's “God”, if not the God of Religion.
But to be fair, no matter what conception of Ultimate Reality we adopt, it seems likely that we can only understand it with our limited human minds by employing some set of metaphors or analogies. This applies to Theism at least as strongly as to Naturalism (though perhaps Theists are more often conscious of the fact that they are applying metaphors to God, than Naturalists are when they speak of Nature). In that sense, we are all in the same boat. If the Naturalist wishes to insist that they are using “Law of Nature” in a purely metaphorical way, which connotes order and rationality but excludes any hint of personality or mind, then I cannot say that they are employing a fundamentally illegitimate methodology. Nearly all of our conceptions are metaphorical to some degree, even about lesser matters.
And if there are some deep questions about what the metaphysical meaning of the Laws are, at least their physical content can be precisely stated in precise mathematical terms. Whereas anyone who has studied Theology knows that hashing out the meaning of a fundamental mind, and predicting its effects, is a far murkier subject.
For these reasons, I believe it is not possible on the basis of Cosmological Arguments alone to decide between Naturalism and Theism. But the balance of probabilities is shifted by other types of arguments.
We might check to see if there is any credible evidence that some god has revealed himself to the world through explicit revelation, supported by manifestations such as miracles, prophecies, or visions. In fact we should do this, but it is not something that can be done from our armchairs (not without the aid of books or the internet anyway!) so let's leave this aside for the time being.
Another set of considerations is Design Arguments. These concern the question of whether the Universe is organized in a way that suggests the existence of an intelligent agent with particular goals. One particular type of Design Argument was invalidated by Darwin, but there are other versions, such as the Fine-Tuning Argument, which I'll discuss in depth at a later time. On the flip side are the Undesign Arguments that the Universe is not the way a divine being would organize it: the most convincing forms of these involve Arguments from Evil. But I don't want to consider Design Arguments here. Not because they are irrelevant, but because they don't have much to do with purely Cosmological considerations.
Instead let me consider what we can learn from the Philosophy of Mind and the Philosophy of Ethics. There is a sense in which these give a continuation of the Cosmological Argument, namely that if we believe there is some Source responsible for everything else that is, then what attributes we should attribute to the source will depend on what kinds of things really exist and therefore proceed from that source. (I say kinds of things, not arrangements of things into patterns; the latter would be more akin to a Design Argument). We need to decide whether it would be possible in principle for those kinds of things to come from that proposed Source or not.
In the process, I will naturally have to make some rather controversial statements. In other words, the plausibility of Theism depends on your background beliefs. I hope that doesn't shock any of my readers too much!
Next: Does God Need a Brain?