1. History

Roman Catholic teachings on heresy first began in 380 AD with the Edict of Thessalonica. Later in the thirteenth century heresy was defined by Thomas Aquinas as "a species of infidelity in men who, having professed the faith of Christ, corrupt its dogmas."^[1] St. Thomas continues; "The right Christian faith consists in giving one's voluntary assent to Christ in all that truly belongs to His teaching. There are, therefore, two ways of deviating from Christianity: the one by refusing to believe in Christ Himself, which is the way of infidelity, common to Pagans and Jews; the other by restricting belief to certain points of Christ's doctrine selected and fashioned at pleasure, which is the way of heretics. The subject-matter of both faith and heresy is, therefore, the deposit of the faith, that is, the sum total of truths revealed in Scripture and Tradition as proposed to our belief by the Church. The believer accepts the whole deposit as proposed by the Church; the heretic accepts only such parts of it as commend themselves to his own approval."^[2]

Some significant controversies of doctrine have risen over the course of history. At times there have been many heresies over single points of doctrine, particularly in regard to the nature of the Trinity, the doctrine of transubstantiation and the immaculate conception.

The Church has always fought in favour of orthodoxy and the Pope's authority. At various times in history, it has had varying degrees of power to resist or punish heretics, once it had defined them. The last case of a heretic being executed was that of the schoolmaster Cayetano Ripoll, accused of deism by the waning Spanish Inquisition and hanged to death 26 July 1826 in Valencia after a two-year trial.^[3] Eight years later in 1834, Spain, the last remaining government to still be providing the Catholic Church with the right to pronounce and effect capital punishment, formally withdrew that right from the Church. The era of such absolute Church authority had lasted some 1,449 years, from 385 AD through to 1834 of the 19th century. The number of people executed as heretics as sentenced by various church authorities is not known; however it most certainly numbers into the several thousands. Coincidentally, the first heretic executed had been a Spaniard, Priscillian; the most notorious organization known for the persecution of heretics had been based in Spain, the Spanish Inquisition, and the last heretic executed had been a Spaniard, Cayetano Ripoll. Thus, the era of the execution of heretics by the Catholic Church had come to an end.

Canon 751 of the Catholic Church's Code of Canon law promulgated by Pope John Paul II in 1983 (abbreviated "C.I.C." for Codex Iuris Canonici), the juridical systematization of ancient law currently binding the world's one billion Catholics, defines heresy as the following: "Heresy is the obstinate denial or doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith." The essential elements of canonical heresy therefore technically comprise 1) obstinacy, or continuation in time; 2) denial (a proposition contrary or contradictory in formal logic to a dogma) or doubt (a posited opinion, not being a firm denial, of the contrary or contradictory proposition to a dogma); 3) after reception of valid baptism; 4) of a truth categorized as being of "Divine and Catholic Faith," meaning contained directly within either Sacred Scripture or Sacred Tradition per Can. 750 par. 1 C.I.C. ("de fide divina") AND proposed as 'de fide divina' by either a Pope having spoken solemnly "ex cathedra" on his own (example: dogmatic definition of the Assumption of the Blessed Virgin Mary in 1950), or defined solemnly by an Ecumenical Council in unison with a Pope (ex: the definition of the Divinity of Christ in the Council of Chalcedon) ("de fide catholica").

2. Formal and Material Heresy

The Catholic Church asserts and teaches that its doctrines are the authoritative understandings of the faith taught by Christ and that the Holy Spirit protects the Church from falling into error when teaching these doctrines. To deny one or more of those doctrines, therefore, is to deny the faith of Christ. Heresy is both the non-orthodox belief itself, and the act of holding to that belief. However, the Church makes several distinctions as to the seriousness of an individual heterodoxy and its closeness to true heresy. Only a belief that directly contravenes an Article of Faith, or that has been explicitly rejected by the Church, is labelled as actual "heresy."

An important distinction is that between formal and material heresy. The difference is one of the heretic's subjective belief about his opinion. The heretic who is aware that his belief is at odds with Catholic teaching and yet continues to cling to his belief pertinaciously is a formal heretic. This sort of heresy is sinful because in this case the heretic knowingly holds an opinion that, in the words of the first edition of the Catholic Encyclopedia, "is destructive of the virtue of Christian faith . . . disturbs the unity, and challenges the Divine authority, of the Church" and "strikes at the very source of faith."^[2] Material heresy, on the other hand, means that the individual is unaware that his heretical opinion denies, in the words of Canon 751, "some truth which is to be believed by divine and Catholic faith." The opinion of a material heretic may produce the same objective results as formal heresy, but because of his ignorance he commits no sin by holding it.Caridi, Cathy. "Can a Pope Commit Heresy? ("Heresy" Defined)". http://canonlawmadeeasy.com/2017/09/28/can-a-pope-commit-heresy-heresy-defined/. Retrieved 18 Oct 2018.  The penalty for a baptised Catholic above the age of 18 who obstinately, publicly, and voluntarily manifests his or her adherence to an objective heresy is automatic excommunication ("latae sententiae") according to Can. 1364 par.1 C.I.C..

A belief that the church has not directly rejected, or that is at variance with less important church teachings, is given the label, sententia haeresi proxima, meaning "opinion approaching heresy." A theological argument, belief, or theory that does not constitute heresy in itself, but which leads to conclusions which might be held to do so, is termed propositio theologice erronea, or "erroneous theological proposition." Finally, if the theological position only suggests but does not necessarily lead to a doctrinal conflict, it might be given the even milder label of sententia de haeresi suspecta, haeresim sapiens, meaning "opinion suspected, or savoring, of heresy."

3. Modern Roman Catholic Response to Protestantism

Well into the 20th century, Catholics defined Protestants as heretics. Thus, Hilaire Belloc, in his time one of the most conspicuous speakers for Catholicism in Britain, was outspoken about the "Protestant heresy". He even defined Islam as being "a Christian heresy", on the grounds that Muslims accept many of the tenets of Christianity but deny the divinity of Christ.

However, in the second half of the century, and especially in the wake of Vatican II, the Catholic Church, in the spirit of ecumenism, tended to diminish the effects of Protestantism as a formal heresy by referring to many Protestants who, as material heretics, "through no fault of their own do not know Christ and his Church",^[4] even though the teachings of Protestantism are indeed formally heretical from a Catholic perspective. Modern usage in ecumenical contexts favors referring to Protestants as "separated brethren". In his book The Meaning of Christian Brotherhood, Pope Benedict XVI wrote that:

The difficulty in the way of giving an answer is a profound one. Ultimately it is due to the fact that there is no appropriate category in Catholic thought for the phenomenon of Protestantism today (one could say the same of the relationship to the separated churches of the East). It is obvious that the old category of ‘heresy’ is no longer of any value. Heresy, for Scripture and the early Church, includes the idea of a personal decision against the unity of the Church, and heresy’s characteristic is pertinacia, the obstinacy of him who persists in his own private way. This, however, cannot be regarded as an appropriate description of the spiritual situation of the Protestant Christian. In the course of a now centuries-old history, Protestantism has made an important contribution to the realization of Christian faith, fulfilling a positive function in the development of the Christian message and, above all, often giving rise to a sincere and profound faith in the individual non-Catholic Christian, whose separation from the Catholic affirmation has nothing to do with the pertinacia characteristic of heresy. Perhaps we may here invert a saying of St. Augustine’s: that an old schism becomes a heresy. The very passage of time alters the character of a division, so that an old division is something essentially different from a new one. Something that was once rightly condemned as heresy cannot later simply become true, but it can gradually develop its own positive ecclesial nature, with which the individual is presented as his church and in which he lives as a believer, not as a heretic. This organization of one group, however, ultimately has an effect on the whole. The conclusion is inescapable, then: Protestantism today is something different from heresy in the traditional sense, a phenomenon whose true theological place has not yet been determined.^[5]

Some of the doctrines of Protestantism that the Catholic Church considers heretical are the belief that the Bible is the only source and rule of faith (sola scriptura), that faith alone can lead to salvation (sola fide), that the Pope does not have universal jurisdiction over the whole Church, that the Catholic Church is not "the sole Church of Christ", and that there is no sacramental and ministerial priesthood received by ordination, but only a universal priesthood of all believers.^[6]