HKB
is one who devises or follows false or new opinions. Therefore heresy is opposed to the truth, on which faith is founded; and consequently it is a species of unbelief.
I answer that, The word heresy as stated in the first objection denotes a choosing. Now choice as stated above (I-II, Q. 13, A. 3) is about things directed to the end, the end being presupposed. Now, in matters of faith, the will assents to some truth, as to its proper good, as was shown above (Q. 4, A. 3): wherefore that which is the chief truth, has the character of last end, while those which are secondary truths, have the character of being directed to the end.
Now, whoever believes, assents to someone's words; so that, in every form of unbelief, the person to whose words assent is given seems to hold the chief place and to be the end as it were; while the things by holding which one assents to that person hold a secondary place. Consequently he that holds the Christian faith aright, assents, by his will, to Christ, in those things which truly belong to His doctrine.
Accordingly there are two ways in which a man may deviate from the rectitude of the Christian faith. First, because he is unwilling to assent to Christ: and such a man has an evil will, so to say, in respect of the very end. This belongs to the species of unbelief in pagans and Jews. Secondly, because, though he intends to assent to Christ, yet he fails in his choice of those things wherein he assents to Christ, because he chooses not what Christ really taught, but the suggestions of his own mind.
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?