Objection 1. It would seem that unbelief is not a sin. For every sin is contrary to nature, as Damascene proves (De Fide Orth. ii, 4). Now unbelief seems not to be contrary to nature; for Augustine says (De Praedest. Sanct. v) that "to be capable to having faith, just as to be capable of having charity, is natural to all men; whereas to have faith, even as to have charity, belongs to the grace of the faithful." Therefore not to have faith, which is to be an unbeliever, is not a sin.
Objection 2. Further, no one sins that which he cannot avoid, since every sin is voluntary. Now it is not in a man's power to avoid unbelief, for he cannot avoid it unless he have faith, because the Apostle says (Romans 10:14): "How shall they believe in Him, of Whom they have not heard? And how shall they hear without a preacher?" Therefore unbelief does not seem to be a sin.
Objection 3. Further, as stated above (I-II, 84, 4), there are seven capital sins, to which all sins are reduced. But unbelief does not seem to be comprised under any of them. Therefore unbelief is not a sin.
On the contrary, Vice is opposed to virtue. Now faith is a virtue, and unbelief is opposed to it. Therefore unbelief is a sin.
I answer that, Unbelief may be taken in two ways: first, by way of pure negation, so that a man be called an unbeliever, merely because he has not the faith. Secondly, unbelief may be taken by way of opposition to the faith; in which sense a man refuses to hear the faith, or despises it, according to Isaiah 53:1: "Who hath believed our report?" It is this that completes the notion of unbelief, and it is in this sense that unbelief is a sin.
If, however, we take it by way of pure negation, as we find it in those who have heard nothing about the faith, it bears the character, not of sin, but of punishment, because such like ignorance of Divine things is a result of the sin of our first parent. If such like unbelievers are damned, it is on account of other sins, which cannot be taken away without faith, but not on account of their sin of unbelief. Hence Our Lord said (John 15:22) "If I had not come, and spoken to them, they would not have sin"; which Augustine expounds (Tract. lxxxix in Joan.) as "referring to the sin whereby they believed not in Christ."
Reply to Objection 1. To have the faith is not part of human nature, but it is part of human nature that man's mind should not thwart his inner instinct, and the outward preaching of the truth. Hence, in this way, unbelief is contrary to nature.
Reply to Objection 2. This argument takes unbelief as denoting a pure negation.
Reply to Objection 3. Unbelief, in so far as it is a sin, arises from pride, through which man is unwilling to subject his intellect to the rules of faith, and to the sound interpretation of the Fathers. Hence Gregory says (Moral. xxxi, 45) that "presumptuous innovations arise from vainglory."
It might also be replied that just as the theological virtues are not reduced to the cardinal virtues, but precede them, so too, the vices opposed to the theological virtues are not reduced to the capital vices.
Source: Summa Theologica (New Advent)