Naturalism is a strange belief. It's one thing to hold that it might be true, and quite another to say that it's definitely true, or almost certainly true. Yet there are committed naturalists, just as there are committed Christians.
I understand there is a kind of symmetry here: naturalists think Christianity is a strange belief. On one level I have to agree: we take it as true that everything in all history turns on the life of a wandering teacher and miracle worker who lived two thousand years ago. If it weren't for a confluence of completely unique and remarkable facts about Jesus Christ, the whole idea would be quite unbelievable.
But nowhere near as much as naturalism, by which in this context I mean the idea that nothing exists in all reality but matter and energy, interacting according to necessity (what we call natural law) and chance (according to the most common interpretations of quantum theory). Now there are other varieties of naturalism that do not take this strict physicalist view and I am not speaking of them here.* To believe in naturalism requires believing in a truly preposterous menagerie of ideas that go with it.
For example, it requires believing that personality, rationality, consciousness, identity, purpose, meaning, worth, and the moral significance of life all came out of a reality that excludes every one of these.
We look back at pre-Copernican views of the cosmos, and we smile at the naïveté of thinking the earth was at the center of everything. But that error has nothing on naturalism today: for naturalism requires believing that the things that make us human are absolutely and completely different from anything that is true of anything else anywhere in all reality. We are absolutely different from everything else that exists, and markedly higher, too, for no discernible reason whatsoever.
Either that, or else naturalism takes unguided evolution to be a “reason;” but there is no reason for evolution. If unguided evolution is true, then it is an explanation, but an explanation is not always a reason, especially when it is driven entirely by randomness and chance, as evolution is: for evolution is only randomness plus natural selection; and natural selection is no creative force: it is only a conserver, never an inventor. It is the survival and reproduction of that which survives and reproduces, and the demise of that which doesn't. It is nothing else but that. To reify natural selection as some kind of creative force would be almost as misguided as to assign to it a sense of purpose.
There is no reason to randomness. Evolution could be an explanation, but it could not be a reason for anything. There is especially no reason for evolution to have made a species like ours. Humanness — that which makes us truly distinct, as already mentioned above — would be a strange thing indeed for randomness to have produced.
Either that, or else naturalism requires believing, as many naturalists say they do, that some or all of these things are not real but illusions. Our conceit of being the highest animal is, as Peter Singer puts it, a disreputable sort of “speciesism.” Consciousness is an illusion, say Paul and Patricia Churchland. Morality is a fraud perpetrated on us by evolution, says Michael Ruse. Free will is fake, says Sam Harris. Thinking doesn't exist, thinks Alex Rosenberg.
And yet Rosenberg thinks. Sam Harris chooses. Michael Ruse has moral values he upholds. The Churchlands wake up in the morning, arising to consciousness. And Singer writes his books and articles to humans as if we have a responsibility for our species and for others: he doesn't make the same demands on dolphins and chimpanzees.
Naturalism requires one to believe humans are special for no reason at all, or else to deny that we are special at all. Either option is odd. Naturalism is a strange belief. It's one thing to hold that it is possibly true (though that strains credulity beyond my personal reach). It's another thing altogether — and one can only pause to contemplate what might be the reason for it — for anyone to think it's definitely true, or almost certainly true.
*I could have used the word physicalism throughout, but I find in my reading that naturalism is the more frequently used term. And to write “strict physicalist naturalism” every time would be awkward.