A cult is a religion or sect considered to be false, unorthodox, or extremist, with members often living outside of conventional society under the direction of a charismatic leader. Conservative Christian authors, especially evangelical Protestants, define a cult as a religion which claims to be in conformance with Biblical truth, yet that is believed to deviate from it based upon Evangelical interpretation.

Walter Martin, the pioneer of the Christian countercult movement, gave in his 1955 book the following definition: “By cultism we mean the adherence to doctrines which are pointedly contradictory to orthodox Christianity and which yet claim the distinction of either tracing their origin to orthodox sources or of being in essential harmony with those sources. Cultism, in short, is any major deviation from orthodox Christianity relative to the cardinal doctrines of the Christian faith.”

Since at least the 1940s, the approach of orthodox, conservative, or fundamentalist Christians was to apply the meaning of cult such that it included those religious groups who used (possibly exclusively) non-standard translations of the Bible, put additional revelation on a similar or higher level than the Bible, or had beliefs and/or practices that were not held by current, mainstream Christianity. This is particularly relevant within Evangelical Christianity, which places biblical sources above all others, and would necessarily consider religious movements that use additional sources from outside the Bible to be deviant and even un-Christian.

The countercult movement asserts that non-Christian faiths are spiritually counterfeit and claims the authority to define "true" Christianity, and thus to define "false" Christianity. Christian apologists who write from within this movement argue that a religious body may be defined as a "cult" if its doctrines involve a denial of the teachings which they hold to be essential Christian doctrine such the doctrine of the Trinity, the person and work of Christ, the person and work of the Holy Spirit or the doctrine of salvation.

Countercult ministries concern themselves mainly with religious groups that regard themselves as Christian, but hold beliefs which they consider to be unorthodox, including The Church of Jesus Christ of Latter-day Saints, the Unification church, Christian Science, and Jehovah's Witnesses, although some also target non-Christian groups, such as Scientology, Islam, Wicca and other Neopagan groups, New Age groups, Buddhism, Hinduism, and other Eastern religions.

Christians have applied theological criteria to assess the teachings of non-orthodox movements throughout church history. The Apostles themselves were involved in counter-cult activity. The Apostle Paul wrote an entire epistle, Galatians, antagonistic to the teachings of a Jewish sect that claimed adherence to the teachings of both Jesus and Moses (Acts 15; Galatians 1:6-10). The Apostle John devoted his first Epistle to countering early protognostic cults that had arisen in the first century, all claiming to be "Christian" (1 John. 2:19).

To learn the history of various cults, what they believe and why they believe it and how they deviate from biblical truth, use the links provided below: