noun
dogmatist
A person who asserts opinions in an arrogant, doctrinaire manner; a relatively uncommon loanword used in intellectual or critical contexts.
彼は議論になるとすぐドグマチストになってしまう。
He quickly turns into a dogmatist when it comes to arguments.
Borrowed from English 'dogmatist', ultimately from Greek 'dogma' (opinion, belief).