zealot

Middle English zelote began as an epithet for Simon the Zealot, before taking on its modern sense in the 1500s–1600s. It comes from Latin zēlōtēs, from Greek ζηλωτής (zēlōtḗs, "emulator, zealous admirer, follower"), itself from ζηλόω (zēlóō, "to emulate, to be jealous") and ultimately ζῆλος (zêlos, "zeal, jealousy").

It started as a specific label for a single follower — Simon the Zealot — and only later widened into a general term for anyone fiercely, sometimes fanatically, devoted to a cause.

It shares its root with "zeal" and "zealous," both tracing back to the same Greek zêlos, so all three words carry that same original charge of passionate — even jealous — devotion.