noun
one who has excessive and uncritical zeal or devotion to a principle, cause, religion, leader, or the like.
His views are extreme and he has only the support of a few fanatics.
She’d become a fanatic about her cause, and no one could argue with her.
adjective
overly zealous or enthusiastic; fanatical.
The fanatic fans climbed onto the stage and were arrested by police.
The radicals displayed a fanatic devotion to their cause.