Objection 1. It would seem that theft is not a mortal sin. For it is written (Proverbs 6:30): "The fault is not so great when a man hath stolen." But every mortal sin is a great fault. Therefore theft is not a mortal sin.
Objection 2. Further, mortal sin deserves to be punished with death. But in the Law theft is punished not by death but by indemnity, according to Exodus 22:1, "If any man steal an ox or a sheep... he shall restore five oxen for one ox, and four sheep for one sheep." Therefore theft is not a mortal sin.
Objection 3. Further, theft can be committed in small even as in great things. But it seems unreasonable for a man to be punished with eternal death for the theft of a small thing such as a needle or a quill. Therefore theft is not a mortal sin.
On the contrary, No man is condemned by the Divine judgment save for a mortal sin. Yet a man is condemned for theft, according to Zechariah 5:3, "This is the curse that goeth forth over the face of the earth; for every thief shall be judged as is there written." Therefore theft is a mortal sin.
I answer that, As stated above, (Question 59, Article 4 and Prima Secundae Partis, Question 72, Article 5), a mortal sin is one that is contrary to charity as the spiritual life of the soul. Now charity consists principally in the love of God, and secondarily in the love of our neighbor, which is shown in our wishing and doing him well. But theft is a means of doing harm to our neighbor in his belongings; and if men were to rob one another habitually, human society would be undone. Therefore theft, as being opposed to charity, is a mortal sin.
Reply to Objection 1. The statement that theft is not a great fault is in view of two cases. First, when a person is led to thieve through necessity. This necessity diminishes or entirely removes sin, as we shall show further on (Question 66, Article 7). Hence the text continues: "For he stealeth to fill his hungry soul." Secondly, theft is stated not to be a great fault in comparison with the guilt of adultery, which is punished with death. Hence the text goes on to say of the thief that "if he be taken, he shall restore sevenfold... but he that is an adulterer... shall destroy his own soul."
Reply to Objection 2. The punishments of this life are medicinal rather than retributive. For retribution is reserved to the Divine judgment which is pronounced against sinners "according to the truth" (Rom. ii, 2). Wherefore, according to the judgment of the present life the death punishment is inflicted, not for every mortal sin, but only for such as inflict an irreparable harm, or again for such as contain some horrible deformity. Hence according to the present judgment the pain of death is not inflicted for theft which does not inflict an irreparable harm, except when it is aggravated by some grave circumstance, as in the case of sacrilege which is the theft of a sacred thing, of peculation, which is theft of common property, as Augustine states (Tract. 1, super Joan.), and of kidnaping which is stealing a man, for which the pain of death is inflicted (Exodus 21:16).
Reply to Objection 3. Reason accounts as nothing that which is little: so that a man does not consider himself injured in very little matters: and the person who takes such things can presume that this is not against the will of the owner. And if a person take such like very little things, he may be proportionately excused from mortal sin. Yet if his intention is to rob and injure his neighbor, there may be a mortal sin even in these very little things, even as there may be through consent in a mere thought.
Source: Summa Theologica (New Advent)