OK, so this looks like a fairly fundamental question.
I’m in the process of writing an essay on the history of Shinto for my Patreon, and that reminded me of just how difficult this question is. I’m not talking about the question of the “real essence” of Shinto; I’m talking about the much more basic question of whether a particular activity is part of Shinto.
For example, is Shugendō Shinto? I think most people these days would say “no”, but it was central to the practices of many jinja, and is, today, part of the training offered by one of the centres authorised by Jinja Honchō.
On the other hand, was the so-called “State Shinto” of pre-war Japan actually Shinto? Most people would say “yes”, but all religious elements were supposed to be purged from it; can something completely non-religious actually be Shinto?
Obviously, your answer to these questions will depend on what you think the essence of Shinto is. If Shinto is essentially religious, then State Shinto was not Shinto, despite claiming to be. If Shinto is defined by an absence of features derived from Buddhism, then Shugendō is not Shinto. On the other hand, your beliefs about the essence of Shinto will depend on how you make these judgements. If you think that State Shinto is Shinto, then belief in the kami is not an essential part of Shinto. If you think that Shugendō is, then the presence of many elements that came from Buddhism is compatible with something being Shinto.
Because the answers to the questions about particular practices are not clear, it is possible to defend almost any theory of the essence of Shinto against counterexamples by saying “that isn’t really Shinto”. There are people who argue that State Shinto wasn’t really Shinto, and the theories of Shinto that back that judgement up do not look that unreasonable in themselves.
Similar questions arise for other religions, and other areas of human activity. There are people who deny that Catholicism is a form of Christianity, and doubts as to whether Mormonism is a kind of Christianity are widespread. When I was studying philosophy of science, I studied the debate over what science was. Nothing is as straightforward as it looks, but “Shinto” is one of the more contested labels; there is very little agreement among the people who study it.
For the purposes of my Patreon essays, I aim, roughly, to explain and explore the things that people distinctively do at and think about jinja in the present day. This is one of the areas of rough consensus; I am not aware of anyone who denies that there is a lot of Shinto going on in and around jinja at the moment. Indeed, I would argue that any theory that does deny that is inadequate, for exactly that reason. Any account of Shinto should explain what is going on at jinja now. Shinto may turn out to be a much more recent phenomenon than you might think, requiring a different word for historical practices (and that is a common position among Western scholars, in particular), but the things that are happening now are Shinto.
I am developing my own theory of the essence of Shinto, but that is too idiosyncratic to go in a Patreon essay. Maybe, one day, I will talk about it here.