Objection 1. It would seem that venial and mortal sin do not differ generically, so that some sins be generically mortal, and some generically venial. Because human acts are considered to be generically good or evil according to their matter or object, as stated above (Question 18, Article 2). Now either mortal or venial sin may be committed in regard to any object or matter: since man can love any mutable good, either less than God, which may be a venial sin, or more than God, which is a mortal sin. Therefore venial and mortal sin do not differ generically.
Objection 2. Further, as stated above (1; 72, 5; 87, 3), a sin is called mortal when it is irreparable, venial when it can be repaired. Now irreparability belongs to sin committed out of malice, which, according to some, is irremissible: whereas reparability belongs to sins committed through weakness or ignorance, which are remissible. Therefore mortal and venial sin differ as sin committed through malice differs from sin committed through weakness or ignorance. But, in this respect, sins differ not in genus but in cause, as stated above (77, 8, ad 1). Therefore venial and mortal sin do not differ generically.
Objection 3. Further, it was stated above (74, 3, ad 3; 10) that sudden movements both of the sensuality and of the reason are venial sins. But sudden movements occur in every kind of sin. Therefore no sins are generically venial.
On the contrary, Augustine, in a sermon on Purgatory (De Sanctis, serm. xli), enumerates certain generic venial sins, and certain generic mortal sins.
I answer that, Venial sin is so called from "venia" [pardon]. Consequently a sin may be called venial, first of all, because it has been pardoned: thus Ambrose says that "penance makes every sin venial": and this is called venial "from the result." Secondly, a sin is called venial because it does not contain anything either partially or totally, to prevent its being pardoned: partially, as when a sin contains something diminishing its guilt, e.g. a sin committed through weakness or ignorance: and this is called venial "from the cause": totally, through not destroying the order to the last end, wherefore it deserves temporal, but not everlasting punishment. It is of this venial sin that we wish to speak now.
For as regards the first two, it is evident that they have no determinate genus: whereas venial sin, taken in the third sense, can have a determinate genus, so that one sin may be venial generically, and another generically mortal, according as the genus or species of an act is determined by its object. For, when the will is directed to a thing that is in itself contrary to charity, whereby man is directed to his last end, the sin is mortal by reason of its object. Consequently it is a mortal sin generically, whether it be contrary to the love of God, e.g. blasphemy, perjury, and the like, or against the love of one's neighbor, e.g. murder, adultery, and such like: wherefore such sins are mortal by reason of their genus. Sometimes, however, the sinner's will is directed to a thing containing a certain inordinateness, but which is not contrary to the love of God and one's neighbor, e.g. an idle word, excessive laughter, and so forth: and such sins are venial by reason of their genus.
Nevertheless, since moral acts derive their character of goodness and malice, not only from their objects, but also from some disposition of the agent, as stated above (18, A4,6), it happens sometimes that a sin which is venial generically by reason of its object, becomes mortal on the part of the agent, either because he fixes his last end therein, or because he directs it to something that is a mortal sin in its own genus; for example, if a man direct an idle word to the commission of adultery. In like manner it may happen, on the part of the agent, that a sin generically mortal because venial, by reason of the act being imperfect, i.e. not deliberated by reason, which is the proper principle of an evil act, as we have said above in reference to sudden movements of unbelief.
Reply to Objection 1. The very fact that anyone chooses something that is contrary to divine charity, proves that he prefers it to the love of God, and consequently, that he loves it more than he loves God. Hence it belongs to the genus of some sins, which are of themselves contrary to charity, that something is loved more than God; so that they are mortal by reason of their genus.
Reply to Objection 2. This argument considers those sins which are venial from their cause.
Reply to Objection 3. This argument considers those sins which are venial by reason of the imperfection of the act.
Source: Summa Theologica (New Advent)