The noun zealot means a fanatic person who has extreme opinions about something, usually about religion or politics, and forcefully tries to influence the opinions of others. 

Synonyms are militant, fanatic, enthusiast, or activist.

The word origins from Late Latin (early 14th century) zelotes and from Greek zēlōtēs meaning “one who is a zealous follower” from zēlos meaning “zeal”. It does have an extended sense of “a fanatical enthusiast” first recorded in the 1630s.

She is professional and intelligent, yes, but she is also one of the worst kinds of zealot.

One of the downside of social media is that zealot people are literally everywhere.

I know a place that is run mostly by zealots.