From Christianity’s beginnings, the Church has been attacked by those introducing false teachings, or heresies.
The Bible warned us this would happen. Paul told his young protégé, Timothy, "For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own likings, and will turn away from listening to the truth and wander into myths" (2 Tim. 4:3–4).
What Is Heresy?
Heresy is an emotionally loaded term that is often misused. It is not the same thing as incredulity, schism, apostasy, or other sins against faith. The Catechism of the Catholic Church states, "Incredulity is the neglect of revealed truth or the willful refusal to assent to it. Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and Catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him" (CCC 2089).
To commit heresy, one must refuse to be corrected. A person who is ready to be corrected or who is unaware that what he has been saying is against Church teaching is not a heretic.
A person must be baptized to commit heresy. This means that movements that have split off from or been influenced by Christianity, but that do not practice baptism (or do not practice valid baptism), are not heresies, but separate religions. Examples include Muslims, who do not practice baptism, and Jehovah’s Witnesses, who do not practice valid baptism.
Finally, the doubt or denial involved in heresy must concern a matter that has been revealed by God and solemnly defined by the Church (for example, the Trinity, the Incarnation, the Real Presence of Christ in the Eucharist, the sacrifice of the Mass, the pope’s infallibility, or the Immaculate Conception and Assumption of Mary).
It is important to distinguish heresy from schism and apostasy. In schism, one separates from the Catholic Church without repudiating a defined doctrine. An example of a contemporary schism is the Society of St. Pius X—the "Lefebvrists" or followers of the late Archbishop Marcel Lefebvre—who separated from the Church in the late 1980s, but who have not denied Catholic doctrines. In apostasy, one totally repudiates the Christian faith and no longer even claims to be a Christian.
With this in mind, let’s look at some of the major heresies of Church history and when they began.