Indeed, what is a heresy? Read all the theological works which treat about heresies, a subject which is the first to present itself for definition, since every theology speaks of the true teaching amidst the surrounding false teachings, that is, heresies, and you will nowhere find anything resembling a definition of heresy.
As a specimen of that complete absence of any semblance of a definition of what is understood by the word "heresy" may serve the opinion on this subject expressed by the learned historian of Christianity, E. de Pressensé, in his Histoire du Dogme, with the epigraph, "Ubi Christus, ibi Ecclesia" (Paris, 1869). This is what he says in his introduction: "Je sais que l'on nous conteste le droit de califier ainsi," that is, to call heresies "les tendances qui furent si vivement combattues par les premiers Pères. La désignation même d'hérésie semble une atteinte portée à la liberté de conscience et de pensée. Nous ne pouvons partager ces scrupules, car ils n'iraient à rien moins qu'à enlever au christianisme tout caractère distinctif."
And after saying that after Constantine the church actually misused its power in defining the dissenters as heretics and persecuting them, he passes judgment on the early times and says:
"L'église est une libre association; il y a tout profit à se séparer d'elle. La polémique contre l'erreur n'a d'autres resources que la pensée et le sentiment. Un type doctrinal uniforme n'a pas encore été élaboré; les divergences secondaires se produisent en Orient et en Occident avec une entière liberté, la théologie n'est point liée à d'invariables formules. Si au sein de cette diversité apparait un fond commun de croyances, n'est-on pas en droit d'y voir non pas un système formulé et composé par les représentants d'une autorité d'école, mais la foi elle même, dans son instinct le plus sûr et sa manifestation la plus spontanée? Si cette même unanimité qui se revèle dans les croyances essentielles, se retrouve pour repousser telles ou telles tendances, ne seront-nous pas en droit de conclure que ces tendances étaient en désaccord flagrant avec les principes fondamentaux du christianisme? Cette présomption ne se transformera-t-elle pas en certitude si nous reconnaissons dans la doctrine universellement repoussée par l'église les traits caractéristiques de l'une des religions du passé? Pour dire que le gnosticisme ou l'ebionitisme sont les formes légitimes de la pensée chrétienne, il faut dire hardiment qu'il n'y a pas de pensée chrétienne, ni de caractère specifique qui la fasse reconnaître. Sous prétexte de l'élargir on la dissent. Personne, au temps de Platon, n'eut osé de couvrir de son nom, une doctrine qui n'eut pas fait place à la théorie des idées, et l'on eut excité les justes moqueries de la Grèce, en voulant faire d'Epicure ou de Zénon un disciple de l'Académie. Reconnaissons donc que s'il existe une religion et une doctrine qui s'appelle le christianisme elle peut avoir ses hérésies."
The whole discussion of the author reduces itself to this, that every opinion which is not in agreement with a code of dogmas professed by us at a given time is a heresy; but at a given time and in a given place people profess something, and this profession of something in some place cannot be a criterion of the truth.
Everything reduces itself to this, that "Ubi Christus, ibi Ecclesia;" but Christ is where we are. Every so-called heresy, by recognizing as the truth what it professes, can in a similar manner find in the history of the churches a consistent explanation of what it professes, using for itself all the arguments of De Pressensé and calling only its own confession truly Christian, precisely what all the heresies have been doing.
The only definition of heresy (the word ἁίρεσις means part) is the name given by an assembly of men to every judgment which rejects part of the teaching, as professed by the assembly. A more particular meaning, which more frequently than any other is ascribed to heresy, is that of an opinion which rejects the church doctrine, as established and supported by the worldly power.
There is a remarkable, little known, very large work (Unpartheyische Kirchen und Ketzer-Historia, 1729), by Gottfried Arnold, which treats directly on this subject and which shows all the illegality, arbitrariness, senselessness, and cruelty of using the word "heresy" in the sense of rejection. This book is an attempt at describing the history of Christianity in the form of a history of the heresies.
In the introduction the author puts a number of questions: (1) regarding those who make heretics (von den Ketzermachern selbst); (2) concerning those who were made heretics; (3) concerning the subjects of heresy; (4) concerning the method of making heretics, and (5) concerning the aims and consequences of making heretics.
In connection with each of these points he puts dozens of questions, answers to which he later gives from the works of well-known theologians, but he chiefly leaves it to the reader himself to make the deduction from the exposition of the whole book. I shall quote the following as samples of these questions, which partly contain the answers. In reference to the fourth point, as to how heretics are made, he says in one of his questions (the seventh): "Does not all history show that the greatest makers of heretics and the masters of this work were those same wise men from whom the Father has hidden His secrets, that is, the hypocrites, Pharisees, and lawyers, or entirely godless and corrupt people?" Questions 20 and 21: "And did not, in the most corrupt times of Christianity, the hypocrites and envious people reject those very men who were particularly endowed by God with great gifts, and who in the time of pure Christianity would have been highly esteemed? And, on the contrary, would not these men, who during the decadence of Christianity elevated themselves above everything and recognized themselves to be the teachers of the purest Christianity, have been recognized, in apostolic times, as the basest heretics and antichristians?"