Good points.
Recall Paul wondering why some claimed to follow Paul, Apollos or Cephas?
I believe unique skills such as oratorical,charisma, intelligence, education,knowledge of scriptures, prophecy, healing and such ATTRACTS people around you. It is not intentional but t happens any way. So here we have primitive Christians gravitating towards these powerful men and Paul noticed it. He was rebuking them for this. This is the first step towards cults
The next step is pride sets in and these individuals certainly feel greater and better than everyone else.
The third step is they start exploiting the immense trust. They increasingly start controlling their followers. The usual tactics such as fear,blackmail,isolation and so forth are used. At this stage, there is usually an urgency to increase whatever drew men to them. here is where miracles and prophecies are faked, scriptures are twisted. It is like a drug
After this point, some followers would readily kill for their prophet. The prophet is ALWAYS right. They actually invent stuff to explain away his/her failures. Very defensive. They have acquired a we-vs-them mentality, you are either with them or lost.
From all this, the quickest test of a cult for me is their AUTHORITY. It is ALWAYS some personality or institution. I know at some point all we had were the apostles witness testimonies. But now we have the scriptures. This is closer to Catholicism and I hope you don't take offence. The idea of a Central Authority on doctrine is impractical, the church is too big for this. And it has proven vulnerable to politics, sin and other human weaknesses. Authority is shared through immense literature that is seldom questioned..sermons,books,magazines.....
The next test is EXCLUSIVITY. Am no universalist who believes EVERYONE will be saved somewhat and all sincerity leads to God. Some people, MANY will perish. What I mean by this test is groups that are quick to elevate themselves onto a pedestal and look down on everyone else as wrong are most likely cults. We have Branhams in evangelical circles, followers of William Branham. They are basically headless just reliving the man's sermons and books (sound familiar?). I have heard that Christ Embassy members are discouraged from reading books not penned by Oyakhilome!
The second last test is CONTROL. How much control does a group exert over its member? In extremes, it goes down to how often married members have sex, whom to marry, what to eat, what to wear.....there is a general fear of admitting control because they would look weak. But they dare not break these rules
The last test is DOCTRINE. There will be some specific beliefs peculiar to a group; they don't shake hands, never drink coffee, transfusion....these idionsyncrasies provide some form of IDENTITY and are used to distinguish the group and enforce the them vs us mentality. Note the doctrines are derived from their authority
Of course nobody will admit to belonging to a cult. A pastor joked that the best definition of a heretik is somebody who believes what you don't. if you belong to a group, it is up to you to dig hard and test the veracity of their teachings from scriptures. A thorough examination of the group's history is in order. Inconsistencies in a group's teachings over the years is a red flag
vooke, it's possible it may have started as a cult. After a certain point in time, especially after the leader has gone, and after a certain "mass" or size is reached, I really don't think it's possible for it to continue as such anymore, at least as far as I understand "cult". That type of absolute control exercised by the likes of Manson, Jones, Koresh etc over a few hundred or a few thousand people is not easy to exercise over a large congregation spread out everywhere, especially once the leader is dead, the one who attracted that loyalty in the first place. Many groups disappear with the death of the leader, while others adapt and somehow reorganize.
In fact many religions started out as something of a cult centered on a sacred figure that grew mainstream over time with increased numbers and diversification of membership, whether that increase was achieved through the attractiveness of the message or even by force. Not necessarily that they all exercised that type of control exercised by the likes of Jones or other ancient religious leaders, many were just a group of people who believed that somebody taught truth. Jesus seemed not to have bothered at least during his life, to build a community around him or even to write anything. He was very simple, though startling in his claims, Buddha was a little bit the same way too. But to the Jews, Christianity was what they would've called a cult (of Jesus of Nazareth) if they had our vocabulary. They used much worse descriptors instead, according to the language of the time. 
Islam may be considered the cult of Muhammad, and Buddhism the cult of Buddha. Hinduism is a conglomeration of the cults of various Hindu sages over the ages, whose wisdom/teaching was recorded in the Vedas, mixed with the natural paganism that existed all over the ancient world (the lower form, or the Hinduism of the masses, that is).
But in general, true Cults like Mungiki are IMHO simply not sustainable in the long run, Historically, and over large congregations. They are like criminal gangs in the way they operate, was seeing this story of a gang of bikers in the USA, a murderous group that lived by the absolute word of one guy whose name escapes me at the moment. They didn't have any theistic beliefs, but to me they looked like a cult nonetheless, complete with a prophet. Even the Mormon church cannot IMO be called a cult today, though I do not think they are Christian but a fresh new religion with roots in Christianity, because they are essentially polytheists (belief in many separate gods) and these gods have material/physical bodies like the ancient Greek gods, Zeus, Venus and the rest. Mormons, for example, believe God had sex with Mary to birth Jesus. Back to the cults: These days many cults show up among Koreans and also in Mormon-country (Utah, USA) for some strange reason. The ones in Korea tend to establish worship of the prophet/founder as God in human flesh, while the ones in Utah seem to have a thing for all children and girls in the group being offered in marriage to the prophet, from very young ages, so he has pick of hundreds or tens of them depending on the size of the group.
