Therefore heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas.
Reply Obj. 1: Choice regards unbelief in the same way as the will regards faith, as stated above.
Reply Obj. 2: Vices take their species from their proximate end, while, from their remote end, they take their genus and cause. Thus in the case of adultery committed for the sake of theft, there is the species of adultery taken from its proper end and object; but the ultimate end shows that the act of adultery is both the result of the theft, and is included under it, as an effect under its cause, or a species under its genus, as appears from what we have said about acts in general (I-II, Q. 18, A. 7). Wherefore, as to the case in point also, the proximate end of heresy is adherence to one's own false opinion, and from this it derives its species, while its remote end reveals its cause, viz. that it arises from pride or covetousness.
Reply Obj. 3: Just as heresy is so called from its being a choosing [*From the Greek hairein, to cut off], so does sect derive its name from its being a cutting off (secando), as Isidore states (Etym. viii, 3). Wherefore heresy and sect are the same thing, and each belongs to the works of the flesh, not indeed by reason of the act itself of unbelief in respect of its proximate object, but by reason of its cause, which is either the desire of an undue end in which way it arises from pride or covetousness, as stated in the second objection, or some illusion of the imagination (which gives rise to error, as the Philosopher states in Metaph. iv; Ed. Did. iii, 5), for this faculty has a certain connection with the flesh, in as much as its act is independent on a bodily organ. _______________________
SECOND ARTICLE [II-II, Q. 11, Art. 2]
Whether Heresy Is Properly About Matters of Faith?
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.
Obj. 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 Gal. 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, Q. 16, A. 8, but it is not to be found in St. Jerome's works.] Therefore heresy is not properly about the matter of faith.
Obj. 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, 51]: "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" (Rom. 1:17). Therefore heresy is about matters of faith, as about its proper matter.