The word cult is etymologically derived from a word for worshipping gods that was related to the idea of cultivation/care (In Spanish and French the name for any worship service is a culto/culte).
It became associated with devotion to a person or thing at some point in the last couple hundred years. (As in “cargo cult” “cult following” of a TV show)
In the US, the designation of a religious sect as a cult can have two meanings from what I’ve seen. Some Christians use cult to refer to any religion that is not an orthodox form of one of the creedal world religions. So Nation of Islam is a Muslim cult, Jehovah’s Witnesses are a Christian cult. Probably a better, more neutral word is sect.
It is also used in the wider sense of an ideological community that exercises coercive control over members. So Scientology for example. I personally prefer this meaning and when I use “cult” or “cult-like” I am referring to the social dynamics of an ideological or religious community.
Signs of cults are charismatic figureheads, founders, or leaders who cannot be questioned and aren’t accountable in a meaningful way to any system of checks and balances. They expect unquestioning obedience from cult members and what they decree is seen as authoritative and binding. Cults often consume all aspects of a person’s life, exercising control over how people spend their money, what relationships are allowed, how they make decisions about things like marriage, procreation, and education, and they often require an extensive time commitment devoted to the cult’s meetings or activities or lifestyle requirements. Cults tend to isolate people from meaningful relationships and networks outside the cult community and control access to information. People are often left with very little recourse if they want to get out because they have been cut off from the outside and often depend on the cult or other cult members for their livelihood.
Some fundamentalist Christian groups function in a very cult-like way, even if they aren’t necessarily that unorthodox in their beliefs. They have authoritarian, spiritually/emotionally manipulative leaders. They effectively isolate their group members (especially women and children) from “corruptive” outside voices, influence, and relationships. They exert tremendous social and emotional pressure on members to conform to the group’s norms, to the point where people feel they cannot act independently or cut ties without significant personal cost or harm. The fact that they are so insular and self-protective means things like domestic abuse, sexual abuse and child abuse/neglect often goes unchecked and the authoritarian ways attract abusive men. So you find things like refusing to educate girls, marrying off young women to older men without real consent, using extreme forms of corporal punishment, outlawing birth control and encouraging families so large that siblings are expected to care for/educate younger siblings and provide significant domestic labor in the home since the parents cannot possibly care for all the children, refusing to let women have any control over money or access to the internet.