The first major theological treatment of heresy was by Thomas Aquinas. He refers explicitly to Gratian as the source of some of his quotations from the fathers, and Gratian could have been his source for others also.
Thomas holds that revelation comes to individuals mainly in two ways, through the Bible and through the teaching of the church. ‘‘The formal object of faith is the First Truth, as manifested in the sacred scriptures and the teaching of the church. Anyone who does not adhere, as to an infallible and divine rule, to the teaching of the church, which proceeds from the First Truth manifested in the sacred scriptures, does not have the habit of faith but holds things that belong to the faith in some other way than by faith... A heretic who pertinaciously disbelieves one article is not ready to follow church teaching in all things - but if not pertinaciously, then he is not a heretic but only one who errs’’ (ST 2-2 q.5 a 3). He refers here to the articles of faith, but the argument could be applied to the secondary objects of faith, such as the proposition that Abraham had two sons. Thomas makes this application later (ST 2-2 q.11 a.2). This thesis, that anyone who pertinaciously disbelieves or doubts even such minor point as that Abraham had two sons has no faith at all, illustrates de Guibert’s statement, ‘‘Tandis qu’Augustin refuse de mettre dans cette categorie d’heretiques ceux qui erreraient sur un detail de la verite reveiee, nous mettons aujourd’hui [1920] sur le meme rang toute negation de verite reveiee, quelqu’en soit I’importance et le lien avec I’ensemble de l’economie chretienne” (de Guibert:381): this development seems to be due to Thomas Aquinas.
Thomas quotes Jerome’s remark that ‘‘heresy’’ comes from the Greek word for choice, and says that heresy is the species of unbelief found in those who profess the Christian faith but corrupt its teachings by choosing to assent not to what Christ really taught but to their own ideas. False opinions in geometry are not heresies, since heresies are in matters belonging to the faith - namely the articles of faith, which belong to faith directly and principally, and the secondary objects of faith, denial of which leads to disbelief in some article of faith. For the secondary objects, implicit faith is enough, and it is possible to err about such matters without being a heretic. Thomas quotes Augustine (from the Decretum, C.24, q.3, c. 29): ‘‘We should by no means class as heretics those who defend a false and perverse opinion in no pertinacious spirit, but seek the truth with careful solicitude, ready to correct their opinion when they have found the truth,’’ because they do not choose against the doctrine of the church. Jerome and Augustine differed on some questions, but neither was a heretic because the questions either did not concern faith or had not yet been determined by the church. Pertinacious defense of an error in a matter determined by the church would be heresy. The authority to decide matters of faith belongs to the pope (for this Thomas quotes Decretum, C.24 q.1 c. 12, Friedberg, vol. 1, col. 970; cf. ST 2-2 q.1 a10, where he refers to Decretum, d.17 c. 5, col. 51); Jerome, Augustine, and other orthodox doctors never defended any error against the authority of the pope (ST 2-2 q.11 a.2 ad 3).
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