Something which can be taken for granted

One word substitute is Postulate.

Doctrine: a belief or set of beliefs held and taught by a Church, political party, or other group.
Maxim: a short, pithy statement expressing a general truth or rule of conduct.
Tenet: a principle or belief, especially one of the main principles of a religion or philosophy.
Postulate: suggest or assume the existence, fact, or truth of (something) as a basis for reasoning, discussion, or belief.