Also See Sincere Cultists, Dispelling The Myths, Could You Be A Victim Of Mind Control?
How To Talk To A Person Involved In A Cult (below)
Definition of a Cult - A cult tends to be a rather small group, committed to a few very highly regarded principles, often led by a very “charismatic” (enthusiastic, dynamic, attractive) person. The cultist tends to view everyone outside the group as unbelievers. The cult is usually marked by a very strong, centralized authority... The cultist usually sees himself as one of the elect few. 
What Is a Cult?
Most people have no real concept of a cult and therefore only think of something Satanic or weird. The public’s general impression is that only weak people with serious psychological problems will join a destructive cult. However this is far from the truth. The difference between the cult and true Christianity lies in belief. Though the average cult member will confess a belief in Jesus Christ, a reliance on the Bible as his authority, and devotion to God as a lifestyle, in reality, all of the terms have been redefined. What appears on the surface to be Christian deviates greatly from what the Scriptures actually teach concerning the key doctrines of the Christian faith.
Cults! A Modern Phenomena?
Surprisingly the answer is no. Cults have been around as long as man has and it is interesting that these same old lies often reappear today with fresh promoters. Satan is powerful and deceptive, but he's not very original. He takes the same old heresies, blows them off, gives them a fresh coat of paint, and then sells them to every succeeding generation, which sees them as something new or Biblical. The truth is they are neither. Often, religious people can only see the facade and do not know the depths of spiritual problems. There were many perfect Biblical example at the time of Jesus.
Paul was also familiar with the Jewish sects or cults including the Sadducees who denied life after death and the Zealots who sought a political answer to spiritual problems. And let's not forget the Pharisees who taught a works based salvation conditioned on obedience to laws.
The Twisting of Scripture
There are many individual powerful practices, which characterize the methods of deception employed by false teachers, but the twisting of Scripture is universal among them. One approach is to give respect to the Bible but teach that it cannot be fully trusted. They teach for example that books must have been lost or that texts were altered. Another approach is to hold the Bible as the authority but actually direct the student's attention primarily to the doctrinal books of leaders. These books usually contain some accurate doctrine, but are laced with the leader's heresy. Bible verses are sprinkled liberally in the books to make it appear that the student is studying the Bible. Another method is to actually rewrite the Bible making changes in the text to fit the group's theology.
Who is Jesus?
Christians today witness many religious groups claiming a belief in Jesus. However, their stand on who Jesus is varies widely and it is often contradictory
Is Jesus really the spirit brother of Lucifer and the offspring of a heavenly mother and father as claimed by Mormonism? (See Section Mormonism)
Is Jesus really the archangel Michael, a created being who died on a stake and never rose bodily form the grave as claimed by the Watchtower? Or is Jesus really God incarnate, both fully God and fully man, who died on the cross as total payment for men's sins?
See The Deity of Christ and Section on The Resurrection
The person who puts their trust in a counterfeit Jesus is involved in a deadly case of mistaken identity. Jesus himself pointed this out when he said, "...if ye believe not that I am he, ye shall die in your sins," (John 8:24).
The Jews obviously believed in a Jesus (he was standing right in front of them), but they did not believe the right things about Jesus. Their Jesus was only a man, a carpenter from Nazareth who had become a radical rabbi. "Who art thou?" (vs. 25). In part, Jesus replied, "Verily, verily, I say unto you, Before Abraham was, I am," (John 8:58). Jesus clearly identified himself as God; the ‘I AM’ of Exodus 3.
This is no minor theological "hair-splitting" but strikes at the very essence of the Gospel and the fundamental differences between Christianity and cults.
"Whom do you say that I am?" This question is as relevant today as it was when Jesus asked it of Peter. And, conflicting answers to this question are even more numerous than in the first century.
Was Jesus Wrong?
The position that four of the largest cults in America share is that the first century Church, the original church founded by Jesus and lead by the Apostles, ceased to be at a given point in time.
If the church ceased to be an organization on the earth, then it necessarily follows that if it is on earth today it must have been restored as some particular point in the past. This is precisely what has happened, according to all four groups. While each cult would date the restoration of the church as a different date, that date being the one which resulted in the founding of their specific organization, each would agree that a total apostasy occurred resulting in the need for a restoration of the Church on earth.
At this point there may be an astute Bible student who is asking the question what about the scriptural passage in which Jesus states, "...upon this rock I will build my church; and the gates of hell shall not prevail against it," (Matthew 16:18). For if Jesus promised that the gates of Hell would not prevail against His church, than there can be no total apostasy. If there is no total apostasy, then there is no need for a restoration. If there is no need for a restoration then the church, which Jesus founded is still on earth today. If the church is still on earth today then the argument about the past 19 centuries possessing no true church is erroneous. [See Upon This Rock]
The Basic Patterns:
Exist in all cults who claim to be Christian. Different groups use different combinations of these, but the more of these methods that are in place and the higher the degree of intensity the more destructive the cult.
ADDING TO THE WORD OF GOD: The first pattern apparent in all cults is the habit of adding to the Bible instead of accepting it as the sole authority of one's faith. Throughout Scripture, God warns against this practice. For example, in Proverbs 30:5-6, the Bible states, "Every word of God is pure: he is a shield unto them that put their trust in him. Add thou not unto his words, lest he reprove thee, and thou be found a liar." [See Sola Scriptura]
SUBTRACTING FROM THE PERSON OF CHRIST: The second and most important pattern to look for is the practice of subtracting from the person of Jesus Christ, usually by denying the fact that He is God. This belief is the cornerstone of the Christian faith and supported by numerous passages of Scripture (John 1:1, John 20:27-28, Revelation 1:17 compared with Isaiah 44:6). Of all the cults, the Jehovah’s Witnesses have done one of the most thorough jobs in creating another Jesus. In The Truth Shall Make You Free, they state, "Being the only begotten Son of God and the 'firstborn of every creature', the Word would be a prince among all other creatures. In this office, he bore another name in heaven, which name is 'Michael'. In other words, Jesus in merely a created being not God the Creator. [See The Deity of Christ ]
MULTIPLICATION OF SALVATION REQUIREMENTS: In Ephesians 2:8-9, the Apostle Paul clearly stated, "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: not of works, lest any man should boast." The Word of God plainly teaches that salvation is a free gift that God offers sinful man. There is nothing anyone can do to earn it, it is gift to be accepted or rejected. They have redefined grace to include conditions, which one must keep in order to receive eternal life and by doing so nullified the meaning of grace for grace is God's undeserving love, which He pours out on mankind. [Also See Section Living The Faith]
DIVIDING THE LOYALTY OF THE FOLLOWER: The individual is led to believe that they are not capable of using their own mind and God-given common sense to discover what is right and wrong, but must instead follow the organization. Leaders cannot be questioned, they claim that God will keep them in order: "If I am wrong, pray for me, but don't you dare question God's anointed!" What they say from the platform is set forth as God's word and will. Questioning them is like questioning God himself. Over and over again, this deceptive practice is found throughout the cults as the followers are told to put their faith in God alone but then are commanded to place their faith in the cult! The sad thing is that many cultists are so deceived that they don't see the division and feel that they are fully trusting God.
Information Control: An inordinate form of censorship imposed by the cult dictates what the person will watch in terms of movies, TV or books that are read, or even persons with whom they will socialize. Any ex-members of the group are strictly off-limits. Anything or anyone critical of the group is to be avoided at all costs. The Watchtower Bible and Tract Society discourages reading religious information published by others or even participating in chat rooms where beliefs are discussed is strongly discouraged. The Society warns about the danger of apostasy via the Internet saying, "We have to realize that many web sites have been created by people with immoral or dishonest intent. And many sites that may not be immoral or dishonest, such as chat groups, are a plain waste of time). If you want your license as a minister in the UPC, you are asked if you have a television in your home. If you answer 'yes', even if it is only for video purposes, your application will be denied at General Headquarters in Hazelwood, Missouri.
Behavior Control: All kinds of peculiar rules, dress codes and standards of conduct (not directly specified in the Bible) are impressed upon the victim, causing them to focus on the outward appearance rather than a gradual internal change of heart affected by Christ. If the person does not abide by these unbiblical prohibitions, they are cast out or shunned by the others. Non-conformity is regarded as a rebellious spirit that needs to come under submission to the leadership. (For Example See United Pentecostal)
Deception comes in many forms. Christians need to be well informed in many areas in order to avoid being deceived and to carry out their God-given responsibility to expose deception (Ephesians 5:11). Christians should not begrudge people of other religions the freedom to believe whatever they wish. People have the right even to be wrong. But Christians have the right and duty to defend their own faith from being attacked and usurped; and anyone should rightfully protest fraud and deception if it is used to promote membership in a religious organization. Christians also have the moral responsibility to expose the fraudulent techniques of cult leaders to both the members in the cult and to the public that the cult is exploiting (2 Peter 2:1-3)
(Also See Judge Not?... one of the four most dangerous mistake any Christian can make
Fraternizing With The Enemy
Interestingly many of the cults cry "persecution" and then indicate that this proves that they are the true church. They often fail to remember that they are the ones who say that the orthodox Christian churches are apostate.
Sadly there has been of a lack of emphasis on teaching sound, foundational doctrine in the church and religious people can easily be fooled. The average Christian’s approach is to expect others to teach them. Again, God warns His church concerning this (Ephesians 4:11-15; Colossians 2:6-7). Most Christian are not familiar with Greek and Hebrew nor the way to study the Scriptures with objective study helps. Many do not even read a whole book in the Bible through for its intended context. Therefore, when a false teacher comes with a plausible argument and presents what appears to be a scholarly approach, the novice is impressed.
See The Christian and Knowledge And Why Christians Are Deceived
 Robert L. (Bob)Deffinbaugh. The Characteristics of a Cult. https://bible.org/users/bob-deffinbaugh