Objection 1. It seems that covetousness is always a mortal sin. For no one is worthy of death save for a mortal sin. But men are worthy of death on account of covetousness. For the Apostle after saying (Romans 1:29): "Being filled with all iniquity... fornication, covetousness [Douay: 'avarice']," etc. adds (Romans 1:32): "They who do such things are worthy of death." Therefore covetousness is a mortal sin.
Objection 2. Further, the least degree of covetousness is to hold to one's own inordinately. But this seemingly is a mortal sin: for Basil says (Serm. super. Luc. xii, 18): "It is the hungry man's bread that thou keepest back, the naked man's cloak that thou hoardest, the needy man's money that thou possessest, hence thou despoilest as many as thou mightest succor."
Now 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.
Objection 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 Corinthians 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 (Article 3) covetousness is twofold. On 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 (66, 6,8). Yet venial sin may occur in this kind of covetousness by reason of imperfection of the act, as stated above (66, 6, ad 3), when we were treating of theft.
In another way covetousness may be take 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 to Objection 1. Covetousness is numbered together with mortal sins, by reason of the aspect under which it is a mortal sin.
Reply to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)