There are two kinds of naturalness: (-a-) maturational and (-b-) practiced naturalness.
Examples for (-a-) are walking or speaking my first language.
Examples for (-b-) are reading, hand-writing or riding a bicycle.
(-b-) takes a lot of training to achieve a higher level of competence , but then the activity is 'natural' to us.

The indoctrination hypothesis dear to many atheists claims that religions are like (-b-) and that religiosity is the result of much training, indoctrination, even 'brainwashing'. But this hypothesis is a myth, and the most convincing evidence for 'maturational naturalness' of religiosity is the historical and geographical ubiquity of religions. Whereas cultural activities like writing or mathematics were invented only a few times, there is no known culture in history without religion (in the broad sense, including forms like animism or shamanism).

If the indoctrination hypothesis was true, religions all over the world and in history would be result of a massive conspiracy: nearly all parents of all times, regions and cultures independently (!) taught their children about ghosts, souls, gods, sacred objects and holy places. This is as if all tribes in history had invented the wheel and writing and math... in the Andes and the jungle of Honduras, on the Plains of North America and the savannah of Africa, in Siberia and the outback of Australia, in the Highlands of Papua and in the Nile valley...

Why should parents of all times and places should have done that?

There is just one logical reason: religions are not like writing or reading, but they are more like languages. Children are naturally prepared to learn about the religion of their tribe just as they are born predisposed to learn the language of their tribe. Of course children have to be taught the specifics of their religion; no input, no competence. But the same is true about languages: no input, no competence.

Children are not "born theists", but the human brain is prepared to acquire all sorts of beliefs, including religious beliefs. What is needed is sufficient input and learning, but not indoctrination.

(See: Robert McCauley's book : "Why religion is natural and science is not" )