I answer that, As stated above (I-II, Q. 74, AA. 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 Obj. 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 Obj. 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. _______________________
THIRD ARTICLE [II-II, Q. 10, Art. 3]
Whether Unbelief Is the Greatest of Sins?
Objection 1: It would seem that unbelief is not the greatest of sins. For Augustine says (De Bapt. contra Donat. iv, 20): "I should hesitate to decide whether a very wicked Catholic ought to be preferred to a heretic, in whose life one finds nothing reprehensible beyond the fact that he is a heretic." But a heretic is an unbeliever. Therefore we ought not to say absolutely that unbelief is the greatest of sins.
Obj. 2: Further, that which diminishes or excuses a sin is not, seemingly, the greatest of sins. Now unbelief excuses or diminishes sin: for the Apostle says (1 Tim. 1:12, 13): "I . . . before was a blasphemer, and a persecutor and contumelious; but I obtained . . . mercy . . . because I did it ignorantly in unbelief." Therefore unbelief is not the greatest of sins.
Obj. 3: Further, the greater sin deserves the greater punishment, according to Deut. 25:2: "According to the measure of the sin shall the measure also of the stripes be." Now a greater punishment is due to believers than to unbelievers, according to Heb. 10:29: "How much more, do you think, he deserveth worse punishments, who hath trodden under foot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified?" Therefore unbelief is not the greatest of sins.