Note that there is a large space of possible materialist theisms (eg The Matrix).

The division of nature into spiritual/physical came much later than the invention of religion. I don't think an ancient Greek could have described his beliefs as supernatural in the way that we understand; the concept of "supernatural" wasn't complete until we had a concept of "the natural" as those things that obeyed the laws of nature.

So, I think saying that "religion has the supernatural at its core" (not that I know what Eliezer means to say, but that's part of what I think he's saying) is a projection of more recent, relatively sophisticated theological ideas onto the entire spectrum of religions.

<EDIT> Heck. I read the link Eliezer gave, and actually that isn't what he's saying, because he's defining "supernatural" in a particular way that probably does include the entire spectrum of existing religions:

By far the best definition I've ever heard of the supernatural is Richard Carrier's: A "supernatural" explanation appeals to ontologically basic mental things, mental entities that cannot be reduced to nonmental entities.

This might not describe Unitarianism. It doesn't describe the Matrix theism, which is part of an underexplored space of "theisms that are not religious". And it may just be historical accident that our theisms are religious, and that our religions are supernatural.

Would you consider someone who believed in an absolute morality, but not in a God or an afterlife or spirits, to be religious, but have no supernatural beliefs? </EDIT>