1990s Japanese Cult
The family is part and parcel of being human. It's potentially a great strength for people, sadly though, it's often a great weakness, and the psychological and behavioral patterns that childhood family life imprint on people makes them easy to manipulate. That is, the childhood needs we have persist into adult life--to be accepted, to belong, to be valued--and give manipulators easy ways to exploit us. As an aside, it's pretty interesting that this really basic aspect of human nature is hardly brought off the shelf for discussion and examination through the whole course of education.
Cults take these basic human needs and relationships and repurpose them. That cults form over and over again shows how strong these needs are and how people will bond with almost anything that fills them. The Aum Shinrikyo is my go-to example, over a relatively short span of time, the group grew to have thousands of members and many millions of dollars in assets in pursuit of what appears to be an agenda that, hopefully, any individual member would have rejected were it presented outside the context of their family substitute.
But really, what's a cult? A cult can be the pupal stage of religion, or civilization. Alternatively, a cult can operate in secret, remaining "outside the city" and form an alternative civilization, which can more readily fill the essential human needs of its participants than the established order, especially for the people who have a basic need to be "outside the city". A cult can operate within an established religion, adhering to a completely different set of beliefs than those practiced by the typical congregant of the religion.