Doctrinaire

[Sarah]

One who would apply to political or other practical concernsthe abstract doctrines or the theories of his own philosophicalsystem; a propounder of a new set of opinions; a dogmatic theorist.Used also adjectively; as, doctrinaire notions.

[Sarah]

adhering inflexibly to a practice or theory

[Sommer]

based on and following fixed beliefs rather than considering practical problems:

[Renato]

Seeking to impose a doctrine in all circumstances without regard to practical considerations.