Fanaticism
Etymology
From Latin "fanaticus" (of a temple, inspired by a god, frenzied), from "fanum" (temple). It first described the ecstatic frenzy of temple worshippers, then any devotion carried to a raving extreme.
noun · /fuh-NAT-i-siz-um/
Excessive, uncritical zeal, especially in religion or politics.
What it names
Names belief so intense it loses proportion and tolerance — devotion without a brake.
Example
His fanaticism about the diet had reached the point where dinner with friends was impossible.
Related idea
Zealotry stresses militant action for a cause; fanaticism stresses the loss of proportion and self-criticism. Fanaticism is the state of mind; zealotry is often its conduct.