Objection 1. It would seem that a religious sins mortally whenever he transgresses the things contained in his rule. For to break a vow is a sin worthy of condemnation, as appears from (1 Timothy 5:1)1,12, where the Apostle says that widows who "will marry have [Vulgate: 'having'] damnation, because they have made void their first faith." But religious are bound to a rule by the vows of their profession. Therefore they sin mortally by transgressing the things contained in their rule.
Objection 2. Further, the rule is enjoined upon a religious in the same way as a law. Now he who transgresses a precept of law sins mortally. Therefore it would seem that a monk sins mortally if he transgresses the things contained in his rule.
Objection 3. Further, contempt involves a mortal sin. Now whoever repeatedly does what he ought not to do seems to sin from contempt. Therefore it would seem that a religious sins mortally by frequently transgressing the things contained in his rule.
On the contrary, The religious state is safer than the secular state; wherefore Gregory at the beginning of his Morals [Epist. Missoria, ad Leand. Episc. i] compares the secular life to the stormy sea, and the religious life to the calm port. But if every transgression of the things contained in his rule were to involve a religious in mortal sin, the religious life would be fraught with danger of account of its multitude of observances. Therefore not every transgression of the things contained in the rule is a mortal sin.
I answer that, As stated above (1, ad 1,2), a thing is contained in the rule in two ways. First, as the end of the rule, for instance things that pertain to the acts of the virtues; and the transgression of these, as regards those which come under a common precept, involves a mortal sin; but as regards those which are not included in the common obligation of a precept, the transgression thereof does not involve a mortal sin, except by reason of contempt, because, as stated above (Article 2), a religious is not bound to be perfect, but to tend to perfection, to which the contempt of perfection is opposed.
Secondly, a thing is contained in the rule through pertaining to the outward practice, such as all external observances, to some of which a religious is bound by the vow of his profession. Now the vow of profession regards chiefly the three things aforesaid, namely poverty, continence, and obedience, while all others are directed to these. Consequently the transgression of these three involves a mortal sin, while the transgression of the others does not involve a mortal sin, except either by reason of contempt of the rule (since this is directly contrary to the profession whereby a man vows to live according to the rule), or by reason of a precept, whether given orally by a superior, or expressed in the rule, since this would be to act contrary to the vow of obedience.
Reply to Objection 1. He who professes a rule does not vow to observe all the things contained in the rule, but he vows the regular life which consists essentially in the three aforesaid things. Hence in certain religious orders precaution is taken to profess, not the rule, but to live according to the rule, i.e. to tend to form one's conduct in accordance with the rule as a kind of model; and this is set aside by contempt. Yet greater precaution is observed in some religious orders by professing obedience according to the rule, so that only that which is contrary to a precept of the rule is contrary to the profession, while the transgression or omission of other things binds only under pain of venial sin, because, as stated above (07, ad 2), such things are dispositions to the chief vows. And venial sin is a disposition to mortal, as stated above (I-II, 88, 3), inasmuch as it hinders those things whereby a man is disposed to keep the chief precepts of Christ's law, namely the precepts of charity.
There is also a religious order, that of the Friars Preachers, where such like transgressions or omissions do not, by their very nature, involve sin, either mortal or venial; but they bind one to suffer the punishment affixed thereto, because it is in this way that they are bound to observe such things. Nevertheless they may sin venially or mortally through neglect, concupiscence, or contempt.
Reply to Objection 2. Not all the contents of the law are set forth by way of precept; for some are expressed under the form of ordinance or statute binding under pain of a fixed punishment. Accordingly, just as in the civil law the transgression of a legal statute does not always render a man deserving of bodily death, so neither in the law of the Church does every ordinance or statute bind under mortal sin; and the same applies to the statutes of the rule.
Reply to Objection 3. An action or transgression proceeds from contempt when a man's will refuses to submit to the ordinance of the law or rule, and from this he proceeds to act against the law or rule. on the other hand, he does not sin from contempt, but from some other cause, when he is led to do something against the ordinance of the law or rule through some particular cause such as concupiscence or anger, even though he often repeat the same kind of sin through the same or some other cause. Thus Augustine says (De Nat. et Grat. xxix) that "not all sins are committed through proud contempt." Nevertheless the frequent repetition of a sin leads dispositively to contempt, according to the words of (Proverbs 18:3), "The wicked man, when he is come into the depth of sins, contemneth."
Source: Summa Theologica (New Advent)