The word cult originally meant a system of ritual practice. It first appeared in the 17th century and meant homage paid to a divinity, from the Latin cultus, worship.
The concept of cult as we understand it goes back to 1932 and is a sociological classification. We didn’t make this up!

You will hear cult members insist that we use this word about them simply because we don’t like them and want to exclude them. But this is not true. There is a scientific classification in sociology of “cult” and using the sociological definition we define Mormonism, the Watch Tower, and others as a cult.
Sociologists first distinguished between three types of religious behaviour: church, sect and mystic.

If “church” is the mainstream body of believers a “sect” is a break-way from that body. Here we get the idea of sectarianism, division. Mysticism goes even further, putting forward the idea of enlightenment, or mystical attainment regardless of faith. Later, church was split into ecclesia and denomination and sect became sect and cult. Cult then came to mean a deviant religious group “deriving their inspiration from outside the predominant culture or denomination.”

Sociologists say that sects are products of religious schism that, nevertheless, maintain a continuity with traditional beliefs and practices while cults arise spontaneously around novel beliefs and practices. It is, then, a legitimate sociological category we are using when we use the term cult.

A Cult is in Error

A cult in our context is a group or organisation that claims to be Christian but that deviates from long-established core Christian teachings. Typically, a cult will deny the deity of Christ, or deny the Trinity in some way, and deny the clear Bible message of salvation by grace alone, through faith alone, in Christ alone.

A typical objection to this is illustrated by an exchange I had with a Mormon on the Reachout Facebook page. A post encouraged a conversation about the faith-based message of the New Testament versus the works-based message of Mormonism. The Mormon commented, ‘We’re not Protestant, get over it.’ He went on to write:

‘The point is you’re loading what’s “biblical” with your own tradition’s, presuppositions which we do not share. It’s disingenuous to assess our beliefs outside of our own paradigm and present it as common sense to do so.’
Without realising it, he had said that the Mormon paradigm is distinct from the traditional Christian world-view, which is the point. I replied:

‘Your first point is, itself, loaded with presuppositions. Every Mormon addressing such a question presupposes that the words of Joseph Smith in Joseph Smith History 1:19 are true, that the faith position from which I come is apostate and corrupt. The OP is meant to draw out a discussion on such fundamental error.

‘You also presume to suppose yours is the paradigm by which questions about your faith should be judged. This doesn’t hold water. If your faith was so foreign that it couldn’t be mistaken for my faith, then you might have a point. But your organisation insists it is Christian, indeed the original Christianity. Mormonism presumes to operate in a territory that is already well established as Christian, both historically and theologically. In effect, you are in my house, but insist that I play by your rules. The discussion must revolve around whether your claim to authentic Christian identity is acceptable in light of long-established Christian doctrines. It is a legitimate question.

‘You also presume to make a distinction between what is biblical and what is tradition. The traditions of the Christian Church are biblical since they are founded on what the Bible says. It is misleading to mark such a distinction without first entering into a discussion of where the established Christian traditions come from. A serious study of the confessions of the church are very helpful here.’

It is essential we remember that the Christian faith is established, issues have been addressed, controversies and challenges have been met, and we know what it is to be a follower of Christ. It is not unreasonable to catechise people who come bringing apparently aberrant teachings and ideas, and to compare them with well-established, biblical truths. The Bible, as well as biblically informed hard fought for Christian doctrines and practices, is the standard; the cult is on trial, not the Bible.

A Cult is Authoritarian

A cult typically claims it alone is God’s official channel on earth, it alone has Godly authority. This lends itself to a high level of control and manipulation of its members.

Members are often isolated in order to achieve a high level of control, of unquestioning loyalty. The best form of isolation is mental and emotional and doesn’t look like isolation at all. It demands loyalty and threatens jeopardy. If you leave one Christian tradition and move to another you are still a Christian, if you leave the cult you are considered lost.

Denying the Trinity

Jehovah’s Witnesses deny the deity of Christ, calling him God’s first created being, through whom he created ‘all other things.’ They say he is the greatest man who ever lived, and in a premortal life was an archangel.

Mormons deny the Trinity in a different way. They accept that Jesus is a god, but teach that God the Father is an exalted man who sits at the head of a pantheon of gods. Each member of the Trinity is a distinct and separate god. Mormonism’s founding narrative begins with Joseph Smith encountering two gods in the woods near his home.

Denying Grace

Cults will use Christian language but they have a different dictionary, different definitions. So, they will us a word like ‘grace’ but will incorrectly mix grace with works. If you appeal to Paul in Ephesians 2; ‘You are saved by grace through faith in Christ,’ they will appeal to the works passages, of which we know there are many; ‘Faith without works is dead.’ James 2:20

Paul is writing as an evangelist, talking about salvation, while James is writing as a pastor, talking about sanctification. The ‘works’ of James are simply the outworking of the faith spoken of by Paul (Ephesians 2:10). Both agree that we are saved by grace through faith in Christ, both agree that our Christian lives are dynamic, issuing in works as we grow in Christ.

Cut and Church

A short answer to the question, what is the difference between a church and a cult is:

A cult is an organisation while a church is a living organism. 1 Peter 2:4,5

A cult proselytises or recruits while a church evangelises. Matthew 28:19,20

A cult uses its members to grow the organisation, a church uses its resources to grow its members. 1 Corinthians 3:1-3

A cult is always right, even when it reverses decisions, changes doctrines, a church recognises the need for vigilance and correction. Acts 15:1-20

If we are to be clear in ministry we must know why we use the terms we do, be able to explain where the dangers lie, and defend our ministry even as we challenge aberrant truth claims.

For a greater details see Defining Cult: Mormonism, a Case Study