Objection 1. It would seem that heresy is not properly about matters of faith. For just as there are heresies and sects among Christians, so were there among the Jews, and Pharisees, as Isidore observes (Etym. viii, 3,4,5). Now their dissensions were not about matters of faith. Therefore heresy is not about matters of faith, as though they were its proper matter.
Objection 2. Further, the matter of faith is the thing believed. Now heresy is not only about things, but also about works, and about interpretations of Holy Writ. For Jerome says on Galatians 5:20 that "whoever expounds the Scriptures in any sense but that of the Holy Ghost by Whom they were written, may be called a heretic, though he may not have left the Church": and elsewhere he says that "heresies spring up from words spoken amiss." [St. Thomas quotes this saying elsewhere, in Sent. iv, D, 13, and III, 16, 8, but it is not to be found in St. Jerome's works.] Therefore heresy is not properly about the matter of faith.
Objection 3. Further, we find the holy doctors differing even about matters pertaining to the faith, for example Augustine and Jerome, on the question about the cessation of the legal observances: and yet this was without any heresy on their part. Therefore heresy is not properly about the matter of faith.
On the contrary, Augustine says against the Manichees [Cf. De Civ. Dei xviii, 1]: "In Christ's Church, those are heretics, who hold mischievous and erroneous opinions, and when rebuked that they may think soundly and rightly, offer a stubborn resistance, and, refusing to mend their pernicious and deadly doctrines, persist in defending them." Now pernicious and deadly doctrines are none but those which are contrary to the dogmas of faith, whereby "the just man liveth" (Romans 1:17). Therefore heresy is about matters of faith, as about its proper matter.
I answer that, We are speaking of heresy now as denoting a corruption of the Christian faith. Now it does not imply a corruption of the Christian faith, if a man has a false opinion in matters that are not of faith, for instance, in questions of geometry and so forth, which cannot belong to the faith by any means; but only when a person has a false opinion about things belonging to the faith.
Now a thing may be of the faith in two ways, as stated above (I, 32, 4; I-II, 1, 6, ad 1; I-II, 2, 5), in one way, directly and principally, e.g. the articles of faith; in another way, indirectly and secondarily, e.g. those matters, the denial of which leads to the corruption of some article of faith; and there may be heresy in either way, even as there can be faith.
Reply to Objection 1. Just as the heresies of the Jews and Pharisees were about opinions relating to Judaism or Pharisaism, so also heresies among Christians are about matter touching the Christian faith.
Reply to Objection 2. A man is said to expound Holy Writ in another sense than that required by the Holy Ghost, when he so distorts the meaning of Holy Writ, that it is contrary to what the Holy Ghost has revealed. Hence it is written (Ezekiel 13:6) about the false prophets: "They have persisted to confirm what they have said," viz. by false interpretations of Scripture. Moreover a man professes his faith by the words that he utters, since confession is an act of faith, as stated above (Question 3, Article 1). Wherefore inordinate words about matters of faith may lead to corruption of the faith; and hence it is that Pope Leo says in a letter to Proterius, Bishop of Alexandria: "The enemies of Christ's cross lie in wait for our every deed and word, so that, if we but give them the slightest pretext, they may accuse us mendaciously of agreeing with Nestorius."
Reply to Objection 3. As Augustine says (Ep. xliii) and we find it stated in the Decretals (xxiv, qu. 3, can. Dixit Apostolus): "By no means should we accuse of heresy those who, however false and perverse their opinion may be, defend it without obstinate fervor, and seek the truth with careful anxiety, ready to mend their opinion, when they have found the truth," because, to wit, they do not make a choice in contradiction to the doctrine of the Church. Accordingly, certain doctors seem to have differed either in matters the holding of which in this or that way is of no consequence, so far as faith is concerned, or even in matters of faith, which were not as yet defined by the Church; although if anyone were obstinately to deny them after they had been defined by the authority of the universal Church, he would be deemed a heretic. This authority resides chiefly in the Sovereign Pontiff. For we read [Decret. xxiv, qu. 1, can. Quoties]: "Whenever a question of faith is in dispute, I think, that all our brethren and fellow bishops ought to refer the matter to none other than Peter, as being the source of their name and honor, against whose authority neither Jerome nor Augustine nor any of the holy doctors defended their opinion." Hence Jerome says (Exposit. Symbol [Among the supposititious works of St. Jerome]): "This, most blessed Pope, is the faith that we have been taught in the Catholic Church. If anything therein has been incorrectly or carelessly expressed, we beg that it may be set aright by you who hold the faith and see of Peter. If however this, our profession, be approved by the judgment of your apostleship, whoever may blame me, will prove that he himself is ignorant, or malicious, or even not a catholic but a heretic."
Source: Summa Theologica (New Advent)