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 (Q. 18, AA. 4, 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 Obj. 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 Obj. 2: This argument considers those sins which are venial from their cause.
Reply Obj. 3: This argument considers those sins which are venial by reason of the imperfection of the act. ________________________
THIRD ARTICLE [I-II, Q. 88, Art. 3]
Whether Venial Sin Is a Disposition to Mortal Sin?
Objection 1: It would seem that venial sin is not a disposition to mortal sin. For one contrary does not dispose to another. But venial and mortal sin are condivided as contrary to one another, as stated above (A. 1). Therefore venial sin is not a disposition to mortal sin.
Obj. 2: Further, an act disposes to something of like species, wherefore it is stated in Ethic. ii, 1, 2, that "from like acts like dispositions and habits are engendered." But mortal and venial sin differ in genus or species, as stated above (A. 2). Therefore venial sin does not dispose to mortal sin.
Obj. 3: Further, if a sin is called venial because it disposes to mortal sin, it follows that whatever disposes to mortal sin is a venial sin. Now every good work disposes to mortal sin; wherefore Augustine says in his Rule (Ep. ccxi) that "pride lies in wait for good works that it may destroy them." Therefore even good works would be venial sins, which is absurd.
On the contrary, It is written (Ecclus. 19:1): "He that contemneth small things shall fall by little and little." Now he that sins venially seems to contemn small things. Therefore by little and little he is disposed to fall away together into mortal sin.