Sectarianism is a category used to identify and classify groups that find it impossible to inhabit the same social world as the rest of society.
Sects are characterized not only because they do not consider themselves part of the wider society (that will be the case of the Haredim) but because they consider themselves the only legitimate representative of that society. They claim that they alone embody the ideals of the larger group because they alone understand God’s will.
A sect usually feels that society has not granted it deserved recognition or that it has granted some other sect undeserved recognition. As a result of these feelings, the sect concludes that its only option is to separate from society and create a perfect community.
In any event: A sect needs an evil reality against which to protest, rail, and define itself. The primary evil of sectarianism is modernity.
A quick and very simplistic example would be what in Hasidic parlance is called the “kosher phone,” but it extends, among other things, to a world-rounded universalistic education.
Several stores that sell smartphones and other digital technology near Mea Shearim, Jerusalem’s largest ultra-Orthodox neighborhood, have been trashed. Customers have been assaulted, and riots have broken out in nearby streets. (See https://www.timesofisrael.com/kosher-phone-freedom-policy-changes-hard-to-swallow-for-ultra-orthodox-rabbis/)
While large Jewish movements such as denominations (Reform, Conservative, Reconstructionist, and others) are devoted to integrating their members into the social order, sects promote divisiveness among Jews.
The specific circumstances of the modern state of Israel have allowed Jewish sects to move into positions of influence and power.
In a seminal work on Jewish sectarianism Bar Ilan emeritus professor Albert I. Baumgarten explains that
In practice, it is difficult to distinguish between a sect and a cult. All sects are cultish, and all cults are sectarian.
Cult and sect overlap; however, a cult is always based on a personality who uses esoteric, pseudo-mystical language and claims to be a ‘spiritual father’ or even ‘elder.’