Reply Obj. 1:  Venial and mortal sin are not condivided in contrariety to one another, as though they were species of one genus, as stated above (A. 1, ad 1), but as an accident is condivided with substance.  Wherefore an accident can be a disposition to a substantial form, so can a venial sin dispose to mortal.

Reply Obj. 2:  Venial sin is not like mortal sin in species; but it is in genus, inasmuch as they both imply a defect of due order, albeit in different ways, as stated (AA. 1, 2).

Reply Obj. 3:  A good work is not, of itself, a disposition to mortal sin; but it can be the matter or occasion of mortal sin accidentally; whereas a venial sin, of its very nature, disposes to mortal sin, as stated. ________________________

FOURTH ARTICLE [I-II, Q. 88, Art. 4]

Whether a Venial Sin Can Become Mortal?

Objection 1:  It would seem that a venial sin can become a mortal sin.  For Augustine in explaining the words of John 3:36:  “He that believeth not the Son, shall not see life,” says (Tract. xii in Joan.):  “The slightest,” i.e. venial, “sins kill if we make little of them.”  Now a sin is called mortal through causing the spiritual death of the soul.  Therefore a venial sin can become mortal.

Obj. 2:  Further, a movement in the sensuality before the consent of reason, is a venial sin, but after consent, is a mortal sin, as stated above (Q. 74, A. 8, ad 2).  Therefore a venial sin can become mortal.

Obj. 3:  Further, venial and mortal sin differ as curable and incurable disease, as stated above (A. 1).  But a curable disease may become incurable.  Therefore a venial sin may become mortal.

Obj. 4:  Further, a disposition may become a habit.  Now venial sin is a disposition to mortal, as stated (A. 3).  Therefore a venial sin can become mortal.

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 (A. 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