Dogmatic
Etymology
From Greek "dogma" ("opinion, belief, decree"), from "dokein" ("to seem, to think"). A dogma was originally a settled teaching, especially of a philosophical school or church. The adjective picked up its critical edge from the sense of a doctrine handed down as beyond question.
adjective · /dawg-MAT-ik/
Asserting opinions as though they were unquestionable truths.
What it names
Use it for the person who states beliefs as settled fact and brooks no doubt — the fault is not having convictions but refusing to test them.
Example
He was so dogmatic about his diet that a shared meal became a lecture.
Related idea
Compare categorical, which describes a statement made without qualification; a claim can be categorical on one occasion, while dogmatic names a habit of mind that treats belief as fact.