Aquinas on Whether Envy is a Mortal Sin: Summa, II, ii, 36, iii.
It is written (Job 5:2): Envy slayeth the little one. Now nothing slays spiritually, except mortal sin. Therefore envy is a mortal sin.
Envy kills the children of God. We are called to become as little children. The slay the growing and maturing little child of God in a man’s soul is to murder God’s offspring. To murder God’s offspring is to murder the Holy Ghost in another man’s life.
Envy is a mortal sin, in respect of its genus. For the genus of a sin is taken from its object; and envy according to the aspect of its object is contrary to charity, whence the soul derives its spiritual life, according to 1 John 3:14: We know that we have passed from death to life, because we love the brethren. Now the object both of charity and of envy is our neighbor’s good, but by contrary movements, since charity rejoices in our neighbor’s good, while envy grieves over it, as stated above (Article 1). Therefore it is evident that envy is a mortal sin in respect of its genus.
Envy is a kind or type of sin. According to the aspect of the object under consideration envy is the vice that stands opposite to the virtue of Charity. The soul derives its spiritual life from the Charity or the Love of God. If the love of God or Charity is the life of God in the soul, then the soul is animated by Charity that is extended to all others. The subject, being filled with Charity, turns to the object. The object is our neighbour’s good. Envy is sorrowful and grieves over our neighbour’s potential and actual good. Envy begrudges the Good’s presence and then perfection in our neighbour’s soul. Envy cannot bear the Good in any other man.
Nevertheless, in every kind of mortal sin we find certain imperfect movements in the sensuality, which are venial sins: such are the first movement of concupiscence, in the genus of adultery, and the first movement of anger, in the genus of murder, and so in the genus of envy we find sometimes even in perfect men certain first movements, which are venial sins.
Due to the fallen condition, even in the man who does not indulge the mortal sin of envy, there might be certain sensual movements of the body that resist goodness in the life of a brother or sister. So envy might be present as a venial sin in the body because of the Fallen constitution. That it is present in the perfect man as a venial sin, which might be better expressed as irritation, unease, and discomfort, does not mean that it is the same as a Mortal Sin. It can be present in sensual potential and yet not be fully perfected as a rational sin.
Some say that since envy is a kind of sorrow, it is a passion of the sensitive appetite. Now there is no mortal sin in the sensuality, but only in the reason, as Augustine declares (De Trin. xii, 12) Therefore envy is not a mortal sin. But I reply that the movement of envy in so far as it is a passion of the sensuality, is an imperfect thing in the genus of human acts, the principle of which is the reason, so that envy of that kind is not a mortal sin. The same applies to the envy of little children who have not the use of reason.
So envy is not a mortal sin before reason is fully developed. However, once reason is developed, the knowledge of begrudging another man his share in God’s Goodness renders the sin a Mortal Sin. That babies reveal a horrifically zelous form of envy cannot be held against them since they reveal merely the presence of Original Sin that is not yet perfected by reason.
Furthermore, According to the Philosopher (Rhet. ii, 9), envy is contrary both to nemesis and to pity, but for different reasons. For it is directly contrary to pity, their principal objects being contrary to one another, since the envious man grieves over his neighbor’s good, whereas the pitiful man grieves over his neighbor’s evil, so that the envious have no pity, as he states in the same passage, nor is the pitiful man envious. On the other hand, envy is contrary to nemesis on the part of the man whose good grieves the envious man, for nemesis is sorrow for the good of the undeserving according to Psalm 72:3: I was envious of the wicked, when I saw the prosperity of sinners. Whereas the envious grieves over the good of those who are deserving of it. Hence it is clear that the former contrariety is more direct than the latter. Now pity is a virtue, and an effect proper to charity: so that envy is contrary to pity and charity.
So envy lies between pity and nemesis. Pity is a passion that grieves over his neighbor’s misfortune or the evil in his life. Nemesis is a passion that grieves over a man’s undeserved and unmerited goodness. So nemesis grieves not over the good man but over the sinner. Envy grieves over the good man and his goodness. Pity is a virtue and is caused by charity. Envy is contrary to the virtues of charity and its pity. Envy is opposed also to nemesis. Envy mourns over the sanctity of the saint. Nemesis mourns over the success of the sinner.
Now envy can be a natural or a spiritual sin. It can be directed against a man’s earthly success or against his spiritual profit. The latter is far more sinful than the former, for in expressing it we wish to deny spiritual transformation and salvation to our neighbors. To sorrow over the Grace of God in another man’s heart is truly a Capital Sin. ‘Hence it is accounted a sin against the Holy Ghost, because thereby a man envies, as it were, the Holy Ghost Himself, Who is glorified in His works.’ To envy the flow of Grace in another man’s heart is to begrudge God the Father, His Word, and the Holy Ghost their rightful place in our neighbor’s life. Such is a sin unto death.