rtal.

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 sin incurs a debt of temporal punishment, as
stated above (Q. 87, AA. 3, 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 (Hom. xxiv in Matth.). 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 (A. 3) with
regard to the two different manners of disposition, whereby venial
sin disposes to mortal sin.

Reply Obj. 1: Augustine is referring to the fact of many venial sin