Objection 1. It would seem that heresy is not a species of unbelief. For unbelief is in the understanding, as stated above (Question 10, Article 2). Now heresy would seem not to pertain to the understanding, but rather to the appetitive power; for Jerome says on Galatians 5:19: [Cf. Decretals xxiv, qu. iii, cap. 27 "The works of the flesh are manifest: Heresy is derived from a Greek word meaning choice, whereby a man makes choice of that school which he deems best." But choice is an act of the appetitive power, as stated above (I-II, 13, 1). Therefore heresy is not a species of unbelief.
Objection 2. Further, vice takes its species chiefly from its end; hence the Philosopher says (Ethic. v, 2) that "he who commits adultery that he may steal, is a thief rather than an adulterer." Now the end of heresy is temporal profit, especially lordship and glory, which belong to the vice of pride or covetousness: for Augustine says (De Util. Credendi i) that "a heretic is one who either devises or follows false and new opinions, for the sake of some temporal profit, especially that he may lord and be honored above others." Therefore heresy is a species of pride rather than of unbelief.
Objection 3. Further, since unbelief is in the understanding, it would seem not to pertain to the flesh. Now heresy belongs to the works of the flesh, for the Apostle says (Galatians 5:19): "The works of the flesh are manifest, which are fornication, uncleanness," and among the others, he adds, "dissensions, sects," which are the same as heresies. Therefore heresy is not a species of unbelief.
On the contrary, Falsehood is contrary to truth. Now a heretic 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, 13, 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 (Question 4, Article 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 to Objection 1. Choice regards unbelief in the same way as the will regards faith, as stated above.
Reply to Objection 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, 18, 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 to Objection 3. Just as heresy is so called from its being a choosing [From the Greek airein [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.
Source: Summa Theologica (New Advent)