What is heresy?
Michael Horton helpfully defines it as “any teaching that directly contradicts the clear and direct witness of the Scriptures on a point of salvific importance.” Heresy is the kind of doctrinal error that is so serious that it redefines the gospel. Error is always costly. It dishonours God and damages the Church. But not all errors are heresies. A heretic is not someone who fails to explain adequately the doctrine of the Trinity, or that Jesus is both fully God and fully man, the nature of the atonement, or justification by faith alone. No, a heretic denies these truths and is fundamentally unsubmissive to apostolic doctrine and authority as it is given in Scripture.
Heresy is not a matter of opinion. We have an objective standard when we want to find out which theological view is correct or orthodox (meaning “right belief”), as Paul shows in 1 Corinthians 15, and which ones are wrong . In the end the fight against heresy is always won by the clear, patient, and thorough exposition of Scripture. Perversely, successful heretics themselves often claim to be truly orthodox and biblical.
Heresy is, however, a matter of choice. It is the choice to believe a different gospel. Augustine said that heretics are men “who were altogether broken off and alienated in matters relating to the actual faith.”
A heretic chooses to tell lies about the God of the Bible because he doesn't want to tell the truth. And a heretic is someone who refuses admonition and is divisive (Titus 3:10-11). Putting it mathematically, heretics take away from the truth of the gospel (and adding to the truth always takes away from it), they divide true churches and aim to multiply new disciples.
Where do heresies come from?
It is vitally important to realise that heresies do not originate in the minds of men and women. Ultimately heresy originates with the devil. When the apostle Paul takes the Corinthian church to task for tolerating false teachers he compares their approach to the deception of Eve by the serpent (2 Cor. 11:3). But the deception in the Garden is more than a useful illustration. The super-apostles at Corinth are the servants of the devil disguising themselves as apostles of Christ.
Similarly Paul warned Timothy about “deceitful spirits and the teachings of demons” (1 Tim. 4:1), and of false teachers who are caught in the snare of the devil (2 Tim. 2:24-25). After all the devil is the father of lies (John 8:44). The connection between other gospels and the demonic, which is integral to a biblical world view, has been largely lost. If it were regained it would keep us from ever thinking that heresies are interesting, intellectually stimulating, tolerable, or in any way benign. Cyprian of Carthage, in the third century, made this insightful comment about heresy and the devil:
There is more need to fear and beware of the Enemy when he creeps up secretly, when he beguiles us by a show of peace and steals forward by those hidden approaches which have earned him the name of the 'Serpent'...He invented heresies and schisms so as to undermine the faith, to corrupt the truth, to sunder our unity. Those whom he failed to keep in the blindness of their old ways he beguiles, and leads them up a new road of illusion.
Or as the late Jaroslav Pelikan put it “renouncing the devil means denouncing heresy.”
Furthermore, it is vitally important to understand that heresy is the takeover of Christianity by an alien worldview. Paul warned the Colossians about “plausible arguments” and those who were trying to take them captive by “philosophy and empty deceit according to human tradition,” (Col. 2:4, 8). Heretics often use the words of the Bible, change their meaning, and hide false ideas under them. The label may still say “Christ,” “salvation,” or “atonement” but the meaning of these words have been radically altered. The early church fathers were alert to this danger. They wrote books to expose the fact that heretics were really saying the same thing as pagan philosophers, only the heretics were dressing up these ideas in Christian language. This deceitfulness makes heresy morally as well as doctrinally wrong.