said to displace the stone that rests on it. Accordingly
an act of sin disposes to something in two ways. First, directly, and
thus it disposes to an act of like species. In this way, a sin
generically venial does not, primarily and of its nature, dispose to
a sin generically mortal, for they differ in species. Nevertheless,
in this same way, a venial sin can dispose, by way of consequence, to
a sin which is mortal on the part of the agent: because the
disposition or habit may be so far strengthened by acts of venial
sin, that the lust of sinning increases, and the sinner fixes his end
in that venial sin: since the end for one who has a habit, as such,
is to work according to that habit; and the consequence will be that,
by sinning often venially, he becomes disposed to a mortal sin.
Secondly, a human act disposes to something by removing an obstacle
thereto. In this way a sin generically venial can dispose to a sin
generically mortal. Because he that commits a sin generically venial,
turns aside from some particular order; and through accustoming his
will not to be subject to the due order in lesser matters, is
disposed not to subject his will even to the order of the last end,
by choosing something that is a mortal sin in its genus.

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.
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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 mo