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, 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