HKB
it is a mortal sin to do an injustice to another, since it is contrary to the love of our neighbor. Much more therefore is all covetousness a mortal sin.
Obj. 3: Further, no one is struck with spiritual blindness save through a mortal sin, for this deprives a man of the light of grace. But, according to Chrysostom [*Hom. xv in the Opus Imperfectum, falsely ascribed to St. Chrysostom], "Lust for money brings darkness on the soul." Therefore covetousness, which is lust for money, is a mortal sin.
On the contrary, A gloss on 1 Cor. 3:12, "If any man build upon this foundation," says (cf. St. Augustine, De Fide et Oper. xvi) that "he builds wood, hay, stubble, who thinks in the things of the world, how he may please the world," which pertains to the sin of covetousness. Now he that builds wood, hay, stubble, sins not mortally but venially, for it is said of him that "he shall be saved, yet so as by fire." Therefore covetousness is some times a venial sin.
I answer that, As stated above (A. 3) covetousness is twofold. In one way it is opposed to justice, and thus it is a mortal sin in respect of its genus. For in this sense covetousness consists in the unjust taking or retaining of another's property, and this belongs to theft or robbery, which are mortal sins, as stated above (Q. 66, AA. 6, 8). Yet venial sin may occur in this kind of covetousness by reason of imperfection of the act, as stated above (Q. 66, A. 6, ad 3), when we were treating of theft.
In another way covetousness may be taken as opposed to liberality: in which sense it denotes inordinate love of riches. Accordingly if the love of riches becomes so great as to be preferred to charity, in such wise that a man, through love of riches, fear not to act counter to the love of God and his neighbor, covetousness will then be a mortal sin. If, on the other hand, the inordinate nature of his love stops short of this, so that although he love riches too much, yet he does not prefer the love of them to the love of God, and is unwilling for the sake of riches to do anything in opposition to God or his neighbor, then covetousness is a venial sin.
Reply Obj. 1: Covetousness is numbered together with mortal sins, by reason of the aspect under which it is a mortal sin.
Reply Obj. 2: Basil is speaking of a case wherein a man is bound by a legal debt to give of his goods to the poor, either through fear of their want or on account of his having too much.
Reply Obj. 3: Lust for riches, properly speaking, brings darkness on the soul, when it puts out the light of charity, by preferring the love of riches to the love of God.
FIFTH ARTICLE [II-II, Q. 118, Art. 5]
Whether Covetousness Is the Greatest of Sins?
Objection 1: It seems that covetousness is the greatest of sins. For it is written (Ecclus. 10:9): "Nothing is more wicked than a covetous man," and the text continues: "There is not a more wicked thing than to love money: for such a one setteth even his own soul to sale." Tully also says (De Offic. i, under the heading, 'True magnanimity is based chiefly on two things'): "Nothing is so narrow or little minded as to love money." But this pertains to covetousness. Therefore covetousness is the most grievous of sins.
Obj. 2: Further, the more a sin is opposed to charity, the more grievous it is. Now covetousness is most opposed to charity: for Augustine says (QQ. 83, qu. 36) that "greed is the bane of charity." Therefore covetousness is the greatest of sins.