Word for a non-mainstream belief accepted as fact by a sub-culture?

Let's say there is a sub-culture which is built on a foundational truth. But that truth is not accepted by the culture as a whole.

Is there a word for that? I was thinking "Core-Value"? But it's only really a core-value if you're in that sub-culture.

Is there a word that could be understood by those inside and outside the sub-culture? For example, if I referred to it as a core-value to someone outside that sub-culture, they wouldn't understand what I was talking about.

A common example would be a religious principle.

A more general term used for any component of a culture or a subculture is a folkway including both its beliefs and its practices. Merriam-Webster defines it as follows:

a mode of thinking, feeling, or acting common to a given group of people; especially: a traditional social custom

The sum total of all of the beliefs of a culture or a subculture is called a worldview. Tenet is a synonym for belief and probably closer to what you are looking for if "belief" is insufficiently technical and precise in its meaning. A worldview is:

A worldview is a network of ultimate beliefs, assumptions, values, and ideas about the universe and our place in it that shapes how a person understands their life and experiences (and the lives and experiences of others) and how that person acts in response.

Ethos mentioned above, is really a synonym of "worldview" and not of a specific belief or tenet.

Some synonyms for tenet include: principle, belief, doctrine, precept, article of faith, axiom, canon, premise, conviction, view, opinion, position, and teaching.

The term dogma has a primarily religious connotation and is also mildly pejorative when not used in strictly technical sense by religious scholars and clergy.

A "core-value" is often call a norm and differs somewhat from a belief as it is a moral or ethical prescription or value, rather than a belief about what something is. A "norm" is defined as:

a standard or pattern, especially of social behavior, that is typical or expected of a group. "the norms of good behavior in the civil service"

The plural of "norm" in this sense is "norms". It can also be called a convention (as in a social convention).

The term "norm" is a preferred term to describe a particular value of a culture or sub-culture in anthropology and sociology and cultural psychology.

Consider tenet.

ODO:

tenet NOUN

A principle or belief, especially one of the main principles of a religion or philosophy:
‘the tenets of classical liberalism’

‘No culture or religion can boast that its tenets are unique.’
‘He was no stranger to the tenets of humanist educational theory.’