Sectarianism
Etymology
From "sect," Latin secta ("a following, a school of thought, a party"), most plausibly from sequi ("to follow") — those who follow a particular leader or doctrine. A folk association with secare ("to cut") is tempting given the divisions sects create, but it is not the accepted root. From "a distinct following" the word grew into the modern sense of factional loyalty hardened into conflict.
noun · /sek-TAIR-ee-uh-niz-um/
Rigid, often hostile loyalty to a particular religious, political, or ideological faction over any wider common good.
What it names
It explains how a shared society can fracture along lines of creed or party until group identity outranks every other claim, including truth.
Example
Once the ceasefire held, the harder task was undoing decades of sectarianism woven into schools and neighborhoods.
Related idea
A schism is the historical event of a group splitting apart; sectarianism is the enduring attitude that keeps the resulting factions at odds. The split happens once; the sectarianism can last for centuries.