“I think it would be baseless to call atheism a religion but may theists call naturalism a religion?”
As I’ve argued before atheism is the positive affirmation that God does not exist. In my mind that’s relatively uncontroversial. And as we’ve all seen atheists can certainly be “religious.” Atheists have spawned churches, community meetups, and many other activities and events that we would associate with religions. But, given this, it is still quite a far cry from calling atheism a religion, as you yourself seem to realize. However, what of naturalism, the philosophical sub-set of the atheistic worldview?
Naturalism is the belief that nature is all that there is and that no supernatural reality exists. This is, however, a rather simplified understanding of naturalism as one would have to take into consideration what nature actually is and what would constitute a supernatural being. Understandably, there are also different strands of naturalism but for now it should suffice to define naturalism as the belief that God does not exist and that theism is false.
We are also aware of “scientific naturalism” which is the claim that there are no entities in addition to those endorsed by contemporary science. This is because since science affirms no supernatural beings, it therefore implies scientific naturalism which itself is taken by many to imply philosophical naturalism. Notable advocates of this faith view would include the likes of Daniel Dennett, Richard Dawkins, and the influential 20th century philosopher Bertrand Russell.
Philosophical naturalism is, however, not a religion. But what the way that it is similar to a religion is that it very much resembles one in the sense that it performs the cognitive functions of a religion. Human beings are meaning seeking creatures and with that comes a host deep existential questions to which a religion typically provides an answer. These are fundamental questions. Questions like what is the meaning of life? what is our position in the universe? What about life after death? What should we do to improve the human position? Is there such a thing as a highest good for human beings? And so on.
Essentially, like any religion, especially of the likes the theistic ones we may be more so familiar with, philosophical naturalism attempts to answer these deep existential questions. To the naturalist does God exists? No. Do objective moral values and duties exist on naturalism? No. Is there life after death? No, and so on. Given this we may conclude that naturalism performs the cognitive function of a religion, and hence can be defined as a quasi-religion. Naturalism appears to be a religion but it isn’t really.