THIS DEFINITION COMES FROM ApologeticsIndex.org:
“The term ‘cult of Christianity‘ is used of a group, church or organization whose central teachings and/or practices are claimed to be biblical or representative of biblical Christianity, but which are in fact unbiblical and not Christian in nature.
THIS ARTICLE IS PROVIDED BY GOTQUESTIONS.ORG:
Question: “What is the definition of a cult?”
Answer: When people hear the word cult, they often think of a group that worships Satan, sacrifices animals, or takes part in evil, bizarre, and pagan rituals. However, in reality, a cult rarely involves such things. In fact, a cult, in the broadest sense of the word, is simply a religious system with particular rites and customs.
Usually, though, a cult is more narrowly defined, and the word refers to an unorthodox sect whose members distort the original doctrines of the religion. In a Christian context, the definition of a cult is, specifically, “a religious group that denies one or more of the fundamentals of biblical truth.” A cult is a group that teaches doctrines that, if believed, will cause a person to remain unsaved. A cult claims to be part of a religion, yet it denies essential truth(s) of that religion. Therefore, a Christian cult will deny one or more of the fundamental truths of Christianity while still claiming to be Christian.
The two most common teachings of Christian cults are that Jesus was not God and that salvation is not by faith alone. A denial of the deity of Christ results in the view that Jesus’ death was insufficient to pay for our sins. A denial of salvation by faith alone results in the teaching that salvation is achieved by our own works. The apostles dealt with cults in the early years of the church: for example, John addresses the teaching of Gnosticism in 1 John 4:1–3. John’s litmus test for godly doctrine was “Jesus Christ has come in the flesh” (verse 2)—a direct contradiction of the Gnostic heresy (cf. 2 John 1:7).
The two most well-known examples of cults today are the Jehovah’s Witnesses and Mormons. Both groups claim to be Christian, yet both deny the deity of Christ and salvation by faith alone. Jehovah’s Witnesses and Mormons believe many things that are in agreement with or similar to what the Bible teaches. However, the fact that they deny the deity of Christ and preach a salvation by works qualifies them as cults. Many Jehovah’s Witnesses, Mormons, and members of other cults are moral people who genuinely believe they hold the truth. As Christians, our hope and prayer must be that many people involved in cults will see through the lies and will be drawn to the truth of salvation through faith in Jesus Christ alone.
This is a very good video explaining what a Christian Cult is. The speaker gives a good definition of what a Cult is and then he names several well known Cults and explains some of their unbiblical beliefs very well. He also gives the biblical stance on why Christians should stand against Cults, Cultic Movements and false teaching in general.
What is a Cult? (Cult Apologetics 101)