Objection 1. It would seem that sloth is not a mortal sin. For every mortal sin is contrary to a precept of the Divine Law. But sloth seems contrary to no precept, as one may see by going through the precepts of the Decalogue. Therefore sloth is not a mortal sin.
Objection 2. Further, in the same genus, a sin of deed is no less grievous than a sin of thought. Now it is not a mortal sin to refrain in deed from some spiritual good which leads to God, else it would be a mortal sin not to observe the counsels. Therefore it is not a mortal sin to refrain in thought from such like spiritual works. Therefore sloth is not a mortal sin.
Objection 3. Further, no mortal sin is to be found in a perfect man. But sloth is to be found in a perfect man: for Cassian says (De Instit. Caenob. x, l) that "sloth is well known to the solitary, and is a most vexatious and persistent foe to the hermit." Therefore sloth is not always a mortal sin.
On the contrary, It is written (2 Corinthians 7:20): "The sorrow of the world worketh death." But such is sloth; for it is not sorrow "according to God," which is contrasted with sorrow of the world. Therefore it is a mortal sin.
I answer that, As stated above (I-II, 88, 1,2), mortal sin is so called because it destroys the spiritual life which is the effect of charity, whereby God dwells in us. Wherefore any sin which by its very nature is contrary to charity is a mortal sin by reason of its genus. And such is sloth, because the proper effect of charity is joy in God, as stated above (Question 28, Article 1), while sloth is sorrow about spiritual good in as much as it is a Divine good. Therefore sloth is a mortal sin in respect of its genus. But it must be observed with regard to all sins that are mortal in respect of their genus, that they are not mortal, save when they attain to their perfection. Because the consummation of sin is in the consent of reason: for we are speaking now of human sins consisting in human acts, the principle of which is the reason. Wherefore if the sin be a mere beginning of sin in the sensuality alone, without attaining to the consent of reason, it is a venial sin on account of the imperfection of the act. Thus in the genus of adultery, the concupiscence that goes no further than the sensuality is a venial sin, whereas if it reach to the consent of reason, it is a mortal sin. So too, the movement of sloth is sometimes in the sensuality alone, by reason of the opposition of the flesh to the spirit, and then it is a venial sin; whereas sometimes it reaches to the reason, which consents in the dislike, horror and detestation of the Divine good, on account of the flesh utterly prevailing over the spirit. On this case it is evident that sloth is a mortal sin.
Reply to Objection 1. Sloth is opposed to the precept about hallowing the Sabbath day. For this precept, in so far as it is a moral precept, implicitly commands the mind to rest in God: and sorrow of the mind about the Divine good is contrary thereto.
Reply to Objection 2. Sloth is not an aversion of the mind from any spiritual good, but from the Divine good, to which the mind is obliged to adhere. Wherefore if a man is sorry because someone forces him to do acts of virtue that he is not bound to do, this is not a sin of sloth; but when he is sorry to have to do something for God's sake.
Reply to Objection 3. Imperfect movements of sloth are to be found in holy men, but they do not reach to the consent of reason.
Source: Summa Theologica (New Advent)