Summa summae S. Thomae, Tomus II

You will ask 1° by what formally and primarily mortal sin and venial sin are constituted and differ.

I answer. Mortal sin and venial sin are formally and primarily constituted and differentiated by this, that mortal sin is simply against the law, inasmuch as it is against the end of the law, which it overturns; not so venial sin. But the end of the law is the charity of God and neighbor, I Tim., I, 5; for all laws aim at this, that we love God with our whole heart, and our neighbor as ourselves. Or, which is the same thing, they formally and primarily differ in that mortal sin turns one away from God as last end, and constitutes the ultimate end in a creature; but venial sin, the order to the last end remaining, makes an inordinateness regarding means. The reason is because, a perfect transgression of the law being posited, or aversion from God as last end, all apprehend mortal sin, and thence follow the exclusion of sanctifying grace and charity, the dissolution of the friendship of God, the death of the soul irreparable of itself, guilt of eternal punishment, and all other things which are attributed to mortal sin and do not befit venial sin.

You will ask 2° whether mortal sin and venial sin differ essentially from each other.

I answer. Affirmatively. Because the malice of mortal sin, as appears from what has been said, consists in a relation of disproportion to the last end; but the malice of venial sin rests in a relation of disproportion to means not absolutely necessary for the end. But these two relations differ specifically. Therefore.

You will ask 3° whether and how mortal sins can be discerned from venial.

I answer. To wish to define certainly each and every one — this is mortal, this venial — is rash, since it surpasses human industry; and Augustine, the Eagle of Doctors, confesses, book XXI de Civit., last chapter, n. 5, notwithstanding his study¹, that he could not attain to this; and in the Enchirid., c. 78, n. 21, he says it must be weighed rather by divine than by human judgment. Nevertheless concerning some things it can be defined certainly, concerning many probabilistically; wherefore three rules are commonly assigned by theologians, the first from Scripture, the second from tradition, the third from natural reason.

The first rule from Scripture. Sins which in the Scriptures are said to exclude from the kingdom of God, to be abominable, detestable, execrable, worthy of death, or have some other grave malediction attached to them, or to which is said Woe! are mortal; those, however, which are expressed in milder terms are venial. Of the former you have examples in Levit., XXIV, 15-21; Prov., VI, 16-19; Isa., I throughout; Rom., I, 18 seqq.; I Cor., VI, 9 et 10; Galat., V, 19-21; Ephes., V, 3 seqq., and elsewhere; of the latter, Prov., X, 19; Matth., XII, 36 et 37, and elsewhere.

The second rule from tradition. Those things which from the common sense of the Fathers, bishops and doctors are held in the Church as mortal must be reckoned mortal; those held as venial must be reckoned venial: because it is justly presumed that God does not desert his Church in a matter of such moment, nor permit her to be deceived by common error, to the ruin of souls.

The third rule from natural reason. Sins which, according to right reason, gravely injure the charity of God and neighbor are mortal; those which injure it lightly, venial. But to discern whether they injure the charity of God and neighbor gravely or lightly, many things must be considered: 1° the nature of the act, whether it admit or not lightness of matter, of which in the following article;

2° the gravity or lightness of the matter, both on the part of the object and on the part of the end and circumstances, in those things which admit it; 3° whether the sin be of its genus mortal or not; 4° the tenor of the words of the law, whether it commands severely and under grave penalty; 5° what was the intention of the legislator in this case; 6° whether the sin was committed with perfect or imperfect deliberation, whether from malice, or from weakness or ignorance. But when consent perfectly or imperfectly deliberated must be reckoned so, I will say in the following article.