Objection 1. It would seem that a circumstance can make a venial sin mortal. For Augustine says in a sermon on Purgatory (De Sanctis, serm. xli) that "if anger continue for a long time, or if drunkenness be frequent, they become mortal sins." But anger and drunkenness are not mortal but venial sins generically, else they would always be mortal sins. Therefore a circumstance makes a venial sin to be mortal.
Objection 2. Further, the Master says (Sentent. ii, D, 24) that delectation, if morose [See 74, 6], is a mortal sin, but that if it be not morose, it is a venial sin. Now moroseness is a circumstance. Therefore a circumstance makes a venial sin to be mortal.
Objection 3. Further, evil and good differ more than venial and mortal sin, both of which are generically evil. But a circumstance makes a good act to be evil, as when a man gives an alms for vainglory. Much more, therefore, can it make a venial sin to be mortal.
On the contrary, Since a circumstance is an accident, its quantity cannot exceed that of the act itself, derived from the act's genus, because the subject always excels its accident. If, therefore, an act be venial by reason of its genus, it cannot become mortal by reason of an accident: since, in a way, mortal sin infinitely surpasses the quantity of venial sin, as is evident from what has been said (72, 5, ad 1; 87, 5, ad 1).
I answer that, As stated above (7, 1; 18, 5, ad 4; A10,11), when we were treating of circumstances, a circumstance, as such, is an accident of the moral act: and yet a circumstance may happen to be taken as the specific difference of a moral act, and then it loses its nature of circumstance, and constitutes the species of the moral act. This happens in sins when a circumstance adds the deformity of another genus; thus when a man has knowledge of another woman than his wife, the deformity of his act is opposed to chastity; but if this other be another man's wife, there is an additional deformity opposed to justice which forbids one to take what belongs to another; and accordingly this circumstance constitutes a new species of sin known as adultery.
It is, however, impossible for a circumstance to make a venial sin become mortal, unless it adds the deformity of another species. For it has been stated above (Article 1) that the deformity of a venial sin consists in a disorder affecting things that are referred to the end, whereas the deformity of a mortal sin consists in a disorder about the last end. Consequently it is evident that a circumstance cannot make a venial sin to be mortal, so long as it remains a circumstance, but only when it transfers the sin to another species, and becomes, as it were, the specific difference of the moral act.
Reply to Objection 1. Length of time is not a circumstance that draws a sin to another species, nor is frequency or custom, except perhaps by something accidental supervening. For an action does not acquire a new species through being repeated or prolonged, unless by chance something supervene in the repeated or prolonged act to change its species, e.g. disobedience, contempt, or the like.
We must therefore reply to the objection by saying that since anger is a movement of the soul tending to the hurt of one's neighbor, if the angry movement tend to a hurt which is a mortal sin generically, such as murder or robbery, that anger will be a mortal sin generically: and if it be a venial sin, this will be due to the imperfection of the act, in so far as it is a sudden movement of the sensuality: whereas, if it last a long time, it returns to its generic nature, through the consent of reason. If, on the other hand, the hurt to which the angry movement tends, is a sin generically venial, for instance, if a man be angry with someone, so as to wish to say some trifling word in jest that would hurt him a little, the anger will not be mortal sin, however long it last, unless perhaps accidentally; for instance, if it were to give rise to great scandal or something of the kind.
With regard to drunkenness we reply that it is a mortal sin by reason of its genus; for, that a man, without necessity, and through the mere lust of wine, make himself unable to use his reason, whereby he is directed to God and avoids committing many sins, is expressly contrary to virtue. That it be a venial sin, is due some sort of ignorance or weakness, as when a man is ignorant of the strength of the wine, or of his own unfitness, so that he has no thought of getting drunk, for in that case the drunkenness is not imputed to him as a sin, but only the excessive drink. If, however, he gets drunk frequently, this ignorance no longer avails as an excuse, for his will seems to choose to give way to drunkenness rather than to refrain from excess of wine: wherefore the sin returns to its specific nature.
Reply to Objection 2. Morose delectation is not a mortal sin except in those matters which are mortal sins generically. In such matters, if the delectation be not morose, there is a venial sin through imperfection of the act, as we have said with regard to anger (ad 1): because anger is said to be lasting, and delectation to be morose, on account of the approval of the deliberating reason.
Reply to Objection 3. A circumstance does not make a good act to be evil, unless it constitute the species of a sin, as we have stated above (18, 5, ad 4).
Source: Summa Theologica (New Advent)