a fanatic
Examples
A fanatic is one who can't change his mind and won't change the subject.
A fanatic is a man who does what he thinks the Lord would do if only He knew the facts of the case.
Defined in psychological terms, a fanatic is a man who consciously over-compensates a secret doubt.
A fanatic is a nut who has something to believe in.
If you see one cold and vehement at the same time, set him down for a fanatic.
Out of Ireland have we come, great hatred, little room, maimed us at the start. I carry from my mother's womb a fanatic heart.