For humans? Or do you mean for human children? I suppose that language is also "unnatural" for human beings, since we are not born speaking a language. Nevertheless, we naturally pick up a language. At some point, a language becomes "second nature" to us, just like riding a bicycle.
The studies actually say that belief in divine concepts is a natural cognitive position for adults as well.
Religious thought and behaviour can be considered part of the natural human capacities, such as music, political systems, family relations or ethnic coalitions.
Religious thoughts seem to be an emergent property of our standard cognitive capacities.
“The mind has myriad distinct belief networks that contribute to making religious claims quite natural to many people.”
Some form of religious thinking seems to be the path of least resistance for our cognitive systems. By contrast, disbelief is generally the result of deliberate, effortful work against our natural cognitive dispositions — hardly the easiest ideology to propagate.
https://www.nature.com/articles/4551038a
This review examines recent research into religious rituals, communication and transmission of religious knowledge, the development of god-concepts in children, and the origins and character of religious concepts in adults. Together, these studies consistently emphasize and support the notion that the cultural phenomena typically labeled as ‘religion’ may be understood as the product of aggregated ordinary cognition.
http://www.cell.com/trends/cognitive-sciences/abstract/S1364-6613(99)01419-9
I remember having to be told that "God" exists when I was little. I didn't have any "natural" concept of God. I was not a natural believer if by that you mean a believer. I'll agree that "God" may have been a simple enough entity to conceptualize at that age because I could see "God" as a sort of person, and God may have been a convenient answer to various questions I might have had.
I dont think the studies are about any particular god but belief in divine and supernatural concepts.
I can believe that little children can't easily conceptualize natural processes in the manner of the "efficient causation" of modern science, and may find conscious intentions easier to understand at their tender age, but I'm not sure why that should be a big deal. All that means is that the natural process of human maturation allows us as adults a greater range of abstract understanding -- and the capacity for abstract understanding is natural to human beings. Scientific understanding is not in any way unnatural for us, but simply represents an accomplishment that may accompany greater brain development.
As one of the papers alludes to even as adults religious thinking seems to be the path of least resistence and disbelief takes more effort to achieve and maintain.
Some form of religious thinking seems to be the path of least resistance for our cognitive systems. By contrast, disbelief is generally the result of deliberate, effortful work against our natural cognitive dispositions — hardly the easiest ideology to propagate.
https://www.nature.com/articles/4551038a
Disbelief may be overcome with filling the mind with more critical thinking but I think belief is a natural state for the human mind. Not everyone can achieve a high level of critical or scientific thinking so they may never be in that position to know.