A cult is a group with abnormal beliefs and practices. The Bible warns us of cult.
Matthew 24:11 And many false prophets shall rise, and shall deceive many.
2 Corinthians 11:13 For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ.
2 Peter 2:1 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.

Why people follow cult leader
Spirit of error is influential. The devil misinterprets the Bible. Confuse and conquer.
Matthew 4:6 And saith unto him, If thou be the Son of God, cast thyself down: for it is written, He shall give his angels charge concerning thee: and in their hands they shall bear thee up, lest at any time thou dash thy foot against a stone.
2 Corinthians 11:4 For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.
1 John 4:6 We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error.

Some people have itching ears that like strange doctrines.
2 Timothy 4:3 For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears;
Hebrews 13:9 Be not carried about with divers and strange doctrines. For it is a good thing that the heart be established with grace; not with meats, which have not profited them that have been occupied therein.

1. Repeat phrases, clauses or sentences of their doctrines, just like advertisements in the TV.
2. When they say they "preach gospel", they actually mean getting people to join their group. They baptize people easily to increase headcounts. They even "preach gospel" to Christians as an attempt to convert Christians to be their followers.
3. When they come to knock at the door to say to "preach gospel", they usually avoid answering question about their church, or they answer that their church has no name and no denomination.
4. Emotionalism is promoted in the group.
5. When someone asks for recommendation of something, they recommend only the things related to them.
6. They put too much emphasis on how other people view them, regardless it is good or bad. They are overprotective over the group. They defend their group instead of Christianity. They cannot tolerate negative comment. Even a simple discussion about theology that does not fit to their idea may be immediately perceived as attacking their group even though their group is not mentioned in the discussion. Below are a few ways they use to protect their group when they lack argument point:
   1. Tell lie. This includes twisting others' word to make others busy in clarifying. They also play dumb by saying they do not understand what others say.
   2. Turn harsh and launch personal attack. They always tell their followers that people from other churches including pastors admire their teaching. So in reality when they face objection about their teaching, they snap and turn hostile.
   3. Drown others by pouring huge amount of information from their publishing or by repeating a few external supportive comments that they have got.
   4. Use irrational arguments or questions to bombard others into surrender. Then they will tell their followers that their knowledge is higher than others.
   5. Repeat the same rhetorical questions again and again. You should create a list of FAQ to avoid wasting time to rethink the answer when confronting them.
   6. They console themselves by saying that their truth attracts persecution. This is called victim playing, in which victim-hood is fabricated.
7. They create imaginary enemy. For example, they say traditional churches give people the feeling of having three Gods, which is untrue. They then lead their followers to criticize this imaginary enemy in their meetings.
8. They like to put up a question and answer this question themselves. They also consider that other people are obligated to answer their question.
9. They have self-contradicting logics. For example, when others say "a=b, b=c, thus a=c", they disagree by saying it should be "c=a". When others say "c=a" is same with "a=c", they agree. But then when others go back to "a=b, b=c, thus a=c", they disagree again. But yet they think that only the people that agree with their doctrines have rational thinking.
10. Their followers are asked to suppress own intelligence but use weird method such as "Spirit" to understand their writing or practice. It is a set of self-defined "logics". That causes their member to have lower reading comprehension and can easily misunderstand others.
11. They frequently quote the Bible without considering the context, either a verse out of a context, or even just a phrase out of a verse.
    
12. They publish their own "bible". They integrate someone's commentary into their bible, giving the impression that the commentary is equal to the word of God. Their "bible" is expensive.
    
13. The followers read doctrinal books published by them only. Reading their books (including their "bible") is perceived as a way to gain reward from their god.
14. They advocate reading Greek and Hebrew texts. They will start quoting such text without considering that other people do not know that language. A reason they do this is to look bombastic. It is like saying I eat "Oryza sativa" instead of I eat "rice".
15. They create as many different doctrines as possible to let their followers think they have more revelations than others. But yet they turn hostile when other people talk about what they have done or perceive them as different. If they are afraid that other people will talk about it, then they should not do it in the first place or else they should bear the consequence. Amazingly, they claim that they are against any different doctrine and will even criticize other cults. What an irony.
16. They create as many jargons as possible and use the jargons frequently assuming everybody understands. When new followers start to use those jargons, they will feel very ecstatic. On the other hand, they condemn some words by saying those words are not found in their "bible". When others use those words, they feel uneasy and try to "correct" others.
17. They practice legalism (character assassination) and work salvation. Salvation is even trivialized to physical technique, e.g. chanting blah blah blah. They try to influence their follower to live near their meeting place. Otherwise they ask their followers to ignore other churches nearby place of living but to commute far to their meeting place. They are afraid of losing followers so the control is deployed by creating closed community. Devoted followers will feel that they are unable to live outside the group.
18. Their meeting is not open to public. Newcomer can only attend their meeting by following a follower. In the meeting, men and women do not sit together.
19. They take attendance by name. If someone does not turn up that day, they may call afterwards. They perceive absentees as weak.
20. They amen together habitually like chanting to every sentence the speaker says in their meeting.
21. They idolize the founder of their group but they deny it. There may even be a new ideology word named after the founder due to the fanatic obedience to the founder's teaching, such as xxxxxxism. They always mention their founder's name, even more than the name of Jesus Christ.
22. They claim themselves as the only true church or the leader of all churches. They think that their god has chosen them so they feel more superior or elite than others.
23. They will not declare group financial statement to followers.
24. They put up roster to order followers to spend more time in the group instead of asking for volunteers (e.g. housecleaning).
25. They advocate finishing reading the whole Bible. The worse thing is they will start using this point to look more superior than normal churches, such as "A small member of ours knows more about the Bible than ten pastors outside".
26. They do not recognize any theological college outside the group so no external pastor is allowed to give sermon.
27. They habitually say "the Bible says" but most of things they say are not of the Bible. When being criticized, they question why the critics are going against the Bible. In other words, they are equaling themselves to the Bible.
28. "If you have to say you are, you are not." The same thing applies for cults. If they always engage in propaganda to tell that they are Christian, then they are not.

In terms of doctrines
A cult cannot be considered as just another Christian denomination because its difference is in terms of basic Christian doctrines, not in terms of church practice. They are not part of the universal church. They should not claim themselves as Christian if they want to deviate from fundamental Christian doctrines. From the completion of Bible in year 96 until the start of Modern Era, all things essential in the Bible have been interpreted so any new interpretation is useless.

Because they believe the wrong thing, so Holy Spirit does not dwell in them. Thus they hate other people who do not agree with them and tell lie to protect their group. Hatred and lie are not fruits of Holy Spirit.

Rejection of any of the following doctrines is heresy:

1. Any person with sin shall be in eternal punishment.
2. God is Trinity.
3. Jesus is fully God and yet fully man.
4. Jesus is uncreated.
5. Jesus was conceived by the Holy Spirit.
6. Jesus was born of a virgin.
7. Jesus is sinless.
8. Jesus died on a cross.
9. Jesus resurrected bodily.
10. Jesus is now at the right side of The Father.
11. The date and time of Jesus' return to this world shall be unknown to anybody.
12. Jesus' return shall be only once.
13. Any living person who wants to be saved can be saved.

Final words
Cults program a special bug on the followers in which the more other people work out to try to get them out, the deeper they sink into the group. Prevention takes a vital role. Use checklist above to identify early the cults that our family and friends may be involved with and give on time advice. Letting them have the knowledge of the correct doctrines also helps.