Thomas Aquinas

“ Further, evil and good differ more than venial and mortal sin, both of which are generically evil. But a circumstance makes a good act to be evil, as when a man gives an alms for vainglory. Much more, therefore, can it make a venial sin to be mortal.
On the contrary, Since a circumstance is an accident, its quantity cannot exceed that of the act itself, derived from the act's genus, because the subject always excels its accident. ”

“ The division of sin into venial and mortal is not a division of a genus into its species which have an equal share of the generic nature: but it is the division of an analogous term into its parts, of which it is predicated, of the one first, and of the other afterwards. Consequently the perfect notion of sin, which Augustine gives, applies to mortal sin. On the other hand, venial sin is called a sin, in reference to an imperfect notion of sin, and in relation to mortal sin: even as an accident is called a being, in relation to substance, in reference to the imperfect notion of being. ”

“ 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. ”

“ A disposition that becomes a habit, is like an imperfect thing in the same species; thus imperfect science, by being perfected, becomes a habit. On the other hand, venial sin is a disposition to something differing generically, even as an accident which disposes to a substantial form, into which it is never changed.
Art. 5 - Whether a circumstance can make a venial sin to be mortal? Objection 1: It would seem that a circumstance can make a venial sin mortal. ”

“ 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. ”

“ Now either mortal or venial sin may be committed in regard to any object or matter: since man can love any mutable good, either less than God, which may be a venial sin, or more than God, which is a mortal sin. Therefore venial and mortal sin do not differ generically. ”

“ If, on the other hand, the hurt to which the angry movement tends, is a sin generically venial, for instance, if a man be angry with someone, so as to wish to say some trifling word in jest that would hurt him a little, the anger will not be mortal sin, however long it last, unless perhaps accidentally ”

“ Length of time is not a circumstance that draws a sin to another species, nor is frequency or custom, except perhaps by something accidental supervening. For an action does not acquire a new species through being repeated or prolonged, unless by chance something supervene in the repeated or prolonged act to change its species, e.g. ”

“ Hence the defect of order to the last end cannot be repaired through something else as a higher principle, as neither can an error about principles. Wherefore such sins are called mortal, as being irreparable. ”