Objection 1. It seems that ingratitude is always a mortal sin. For one ought to be grateful to God above all. But one is not ungrateful to God by committing a venial sin: else every man would be guilty of ingratitude. Therefore no ingratitude is a venial sin.
Objection 2. Further, a sin is mortal through being contrary to charity, as stated above (Question 24, Article 12). But ingratitude is contrary to charity, since the debt of gratitude proceeds from that virtue, as stated above (106, 1, ad 3; 6, ad 2). Therefore ingratitude is always a mortal sin.
Objection 3. Further, Seneca says (De Benef. ii): "Between the giver and the receiver of a favor there is this law, that the former should forthwith forget having given, and the latter should never forget having received." Now, seemingly, the reason why the giver should forget is that he may be unaware of the sin of the recipient, should the latter prove ungrateful; and there would be no necessity for that if ingratitude were a slight sin. Therefore ingratitude is always a mortal sin.
Objection 4.On the contrary, No one should be put in the way of committing a mortal sin. Yet, according to Seneca (De Benef. ii), "sometimes it is necessary to deceive the person who receives assistance, in order that he may receive without knowing from whom he has received." But this would seem to put the recipient in the way of ingratitude. Therefore ingratitude is not always a mortal sin.
I answer that, As appears from what we have said above (Article 2), a man may be ungrateful in two ways: first, by mere omission, for instance by failing to recognize the favor received, or to express his appreciation of it or to pay something in return, and this is not always a mortal sin, because, as stated above (Question 106, Article 6), the debt of gratitude requires a man to make a liberal return, which, however, he is not bound to do; wherefore if he fail to do so, he does not sin mortally. It is nevertheless a venial sin, because it arises either from some kind of negligence or from some disinclination to virtue in him. And yet ingratitude of this kind may happen to be a mortal sin, by reason either of inward contempt, or of the kind of thing withheld, this being needful to the benefactor, either simply, or in some case of necessity.
Secondly, a man may be ungrateful, because he not only omits to pay the debt of gratitude, but does the contrary. This again is sometimes mortal and sometimes a venial sin, according to the kind of thing that is done.
It must be observed, however, that when ingratitude arises from a mortal sin, it has the perfect character of ingratitude, and when it arises from venial sin, it has the imperfect character.
Reply to Objection 1. By committing a venial sin one is not ungrateful to God to the extent of incurring the guilt of perfect ingratitude: but there is something of ingratitude in a venial sin, in so far as it removes a virtuous act of obedience to God.
Reply to Objection 2. When ingratitude is a venial sin it is not contrary to, but beside charity: since it does not destroy the habit of charity, but excludes some act thereof.
Reply to Objection 3. Seneca also says (De Benef. vii): "When we say that a man after conferring a favor should forget about it, it is a mistake to suppose that we mean him to shake off the recollection of a thing so very praiseworthy. When we say: He must not remember it, we mean that he must not publish it abroad and boast about it."
Reply to Objection 4. He that is unaware of a favor conferred on him is not ungrateful, if he fails to repay it, provided he be prepared to do so if he knew. It is nevertheless commendable at times that the object of a favor should remain in ignorance of it, both in order to avoid vainglory, as when Blessed Nicolas threw gold into a house secretly, wishing to avoid popularity: and because the kindness is all the greater through the benefactor wishing not to shame the person on whom he is conferring the favor.
Source: Summa Theologica (New Advent)