I answer that, The fact of a venial sin becoming a mortal sin may be understood in three ways. First, so that the same identical act be at first a venial, and then a mortal sin. This is impossible: because a sin, like any moral act, consists chiefly in an act of the will: so that an act is not one morally, if the will be changed, although the act be continuous physically. If, however, the will be not changed, it is not possible for a venial sin to become mortal.
Secondly, this may be taken to mean that a sin generically venial, becomes mortal. This is possible, in so far as one may fix one’s end in that venial sin, or direct it to some mortal sin as end, as stated above (Article 2).
Thirdly, this may be understood in the sense of many venial sins constituting one mortal sin. If this be taken as meaning that many venial sins added together make one mortal sin, it is false, because all the venial sins in the world cannot incur a debt of punishment equal to that of one mortal sin. This is evident as regards the duration of the punishment, since mortal sin incurs a debt of eternal punishment, while venial sin incurs a debt of temporal punishment, as stated above (q87, aa3&5). It is also evident as regards the pain of loss, because mortal sins deserve to be punished by the privation of seeing God, to which no other punishment is comparable, as Chrysostom states. It is also evident as regards the pain of sense, as to the remorse of conscience; although as to the pain of fire, the punishments may perhaps not be improportionate to one another.
If, however, this be taken as meaning that many venial sins make one mortal sin dispositively, it is true, as was shown above (Article 3) with regard to the two different manners of disposition, whereby venial sin disposes to mortal sin.