What defines a cult? If you were to write a work of fiction within which the supernatural or otherwise impossible exists, that is simply a work of fantasy or science-fiction. If you were to go on to say that it's true, you don't actually believe that but you say that, it becomes the occult. This is like astrology, ESP, ghosts, or UFO's. If you then organize the believers into an organization with official members, it becomes a cult. However, if you have all that, and just that, people don't think of it as a true cult. MUFON is an organization with official membership, devoted to promoting belief in UFO's. However, since all you have to do is profess belief in UFO's, and pay membership dues, in exchange for which you get a magazine, and can attend meetings, people think of it more as a club or an organization than a cult. In order to be a true cult in the full sense, you must have social traditions, expectations and taboos. In order to be a member, you have to wear certain clothes, wear your hair in a certain way, honor certain holidays, or go through a variety of rituals and ceremonies in a pre-established highly ritualized way. They must have customs that define them as a social group separate from the rest of society. It's obvious who is a member of the group and who isn't. It doesn't have to be clothing or any specific thing, but there has to be enough to separate them. The members identify with the other members of the group more than anyone else. They think of themselves more as members of that group than any other group. The group defines their identity. They need enough rules for the group to be the primary social group for the members. They might not say what you can wear but might say that you must be baptized and then be a vegetarian or whatever.
All groups are defined by a social structure and social practice. Americans identify with elaborate rituals such as presidential inaugurations and the Fourth of July. There are small groups with very different social expectations and taboos than the rest of society, such as nudists. However, these are not cults because they aren't based on a work of fiction. With a true full cult, you have a body of fiction involving the supernatural, an established organization with structural hierarchy, and a collection od social traditions, expectations, and taboos. However, that definition would just as easily apply to large established religions. How do you differentiate between cults and religions? Cults have a small number of members and beliefs very different from the mainstream religions. If it's an offshoot of a mainstream religion, it's called a sect. Christianity began as a Jewish sect, became a cult, and finally became a religion. Mormanism began as a Christian sect, became a cult, and then became a religion only in the 20th Century. Scientology is a cult but if it were much larger, it would be a religion. Scientology is about as large as a cult can be without being a religion. Seventh Day Adventists and Jehovah Witnesses don't have many members but are so similar to mainstream Christianity, they don't count as cults. In order to be a cult, they must have very different beliefs. Occasionally, a religion can turn back into a cult. In the 6th Century B.C., Zoroastrianism was a mainstream religion, but today, it's a cult. Some cults have a tiny number of members. The Branch Davidians had about 200 members. Heaven's Gate had only 40 members. However if there is only one member, such as Ralph Waldo Emerson, that's not a cult either. It's a lone nut.
If you have a large group of people, you need a large complex organization with a hierarchy leaders. However, if you have a small group of people, you can have a very simple organization. Often this simple organization takes the form of a single cult leader and a group of followers. When Mormonism began, it was small enough that all you needed was Joseph Smith himself and a group of followers. As it grew larger, it needed more organizational structure, until today it is so large, it has a vast complex organizational structure and hierarchy. When you have a small group of followers and one leader, usually the founder, the followers usually view the leader as a Messiah. The leader ends up having surprising control over the followers. This often goes to the leader's head, and he ends up being intoxicated by the amount of power he has over these people. You end up with a cycle in which the more the leader is worshipped, the more it feeds his ego, which causes him to exert more control over the members, which feeds his ego further, etc. As time goes on, he must exert increasing amounts of control in order to get the same power trip or rush. You don't have this cycle in all cults. Also, it's not limited to cults. Charles Manson's group was not a cult because there was no work of fiction involved, and it had nothing to do with religion, but you had the same dynamic of a group of people infatuated with a leader which encouraged to control them further, which made him seem more exalted in their minds, which fed his ego further, etc. Actually, towards the end, Charles Manson, did gain a quasi-religious quality simply because if someone is that exalted and adulated, it takes on a quasi-religious tone.
David Koresh used his power to molest children. The ultimate power trip is to have people take their own lives, which is what Jim Jones, David Koresh, and Marshall Applewhite did. Of course, Jim Jones and David Koresh, unlike Marshall Applewhite, committed mass murder. You could say that Marshall Applewhite was the most successful cult leader since he persuaded all of his people to commit suicide without having to kill anyone. Of course, he had far fewer people to convince than either Jim Jones or David Koresh. Marshall Applewhite probably decided within his own mind to commit suicide, perhaps because he thought he was dying of cancer, and wanted to take his people with him. This was probably the same situation as David Koresh, who decided that he would rather die than be captured by the police. In all of these cases, the cult members felt that both the great leader and the other members would be profoundly disappointed in them if they didn't take their own lives, and they cared deeply what the great leaders and other members thought of them. Also, without the great leader, their lives wouldn't be worth living anyway. Although they are fewer in number, there are female cult leaders. There is Elizabeth Clare Prophet of the "Church Universal and Triumphant" in Montana. There was a woman in Ukraine who called herself "Mary Divine Christ".
I always liked Heaven's Gate because they had an extremely interesting work of fiction, as opposed to say, the Branch Davidians who simply believed in the Bible except that the Second Comming had already taken place and Jesus' current alias was David Koresh. Heaven's Gate was similar to Christianity in that they were apocalyptic, millennial, messianic, believed the body was inferior to the soul, that after you die you go to Heaven, and that there will be an end to the world followed by paradise. However this was heavily augmented by UFO's. Like other UFO groups, they said that there was a UFO following Hale-Bopp Comet. They were also influenced by astrology.
What follows is something I wrote which I think would be an interesting work of fiction for a cult. However, I don't advocate that a cult actually use this.