Objection 1. It would seem that unbelief is not in the intellect as its subject. For every sin is in the will, according to Augustine (De Duabus Anim. x, xi). Now unbelief is a sin, as stated above (Article 1). Therefore unbelief resides in the will and not in the intellect.
Objection 2. Further, unbelief is sinful through contempt of the preaching of the faith. But contempt pertains to the will. Therefore unbelief is in the will.
Objection 3. Further, a gloss [Augustine, Enchiridion lx.] on 2 Corinthians 11:14 "Satan... transformeth himself into an angel of light," says that if "a wicked angel pretend to be a good angel, and be taken for a good angel, it is not a dangerous or an unhealthy error, if he does or says what is becoming to a good angel." This seems to be because of the rectitude of the will of the man who adheres to the angel, since his intention is to adhere to a good angel. Therefore the sin of unbelief seems to consist entirely in a perverse will: and, consequently, it does not reside in the intellect.
On the contrary, Things which are contrary to one another are in the same subject. Now faith, to which unbelief is opposed, resides in the intellect. Therefore unbelief also is in the intellect.
I answer that, As stated above (I-II, 74, 1,2), sin is said to be in the power which is the principle of the sinful act. Now a sinful act may have two principles: one is its first and universal principle, which commands all acts of sin; and this is the will, because every sin is voluntary. The other principle of the sinful act is the proper and proximate principle which elicits the sinful act: thus the concupiscible is the principle of gluttony and lust, wherefore these sins are said to be in the concupiscible. Now dissent, which is the act proper to unbelief, is an act of the intellect, moved, however, by the will, just as assent is.
Therefore unbelief, like faith, is in the intellect as its proximate subject. But it is in the will as its first moving principle, in which way every sin is said to be in the will.
Hence the Reply to the First Objection is clear.
Reply to Objection 2. The will's contempt causes the intellect's dissent, which completes the notion of unbelief. Hence the cause of unbelief is in the will, while unbelief itself is in the intellect.
Reply to Objection 3. He that believes a wicked angel to be a good one, does not dissent from a matter of faith, because "his bodily senses are deceived, while his mind does not depart from a true and right judgment" as the gloss observes [Augustine, Enchiridion lx]. But, according to the same authority, to adhere to Satan when he begins to invite one to his abode, i.e. wickedness and error, is not without sin.
Source: Summa Theologica (New Advent)