Objection 1. It seems that disobedience is not a mortal sin. For every sin is a disobedience, as appears from Ambrose's definition given above (104, 2, Objection 1). Therefore if disobedience were a mortal sin, every sin would be mortal.
Objection 2. Further, Gregory says (Moral. xxxi) that disobedience is born of vainglory. But vainglory is not a mortal sin. Neither therefore is disobedience.
Objection 3. Further, a person is said to be disobedient when he does not fulfil a superior's command. But superiors often issue so many commands that it is seldom, if ever, possible to fulfil them. Therefore if disobedience were a mortal sin, it would follow that man cannot avoid mortal sin, which is absurd. Wherefore disobedience is not a mortal sin.
On the contrary, The sin of disobedience to parents is reckoned (Romans 1:30; 2 Timothy 3:2) among other mortal sins.
I answer that, As stated above (24, 12; I-II, 72, 5; I-II, 88, 1), a mortal sin is one that is contrary to charity which is the cause of spiritual life. Now by charity we love God and our neighbor. The charity of God requires that we obey His commandments, as stated above (Question 24, Article 12). Therefore to be disobedient to the commandments of God is a mortal sin, because it is contrary to the love of God.
Again, the commandments of God contain the precept of obedience to superiors. Wherefore also disobedience to the commands of a superior is a mortal sin, as being contrary to the love of God, according to Romans 13:2, "He that resisteth the power, resisteth the ordinance of God." It is also contrary to the love of our neighbor, as it withdraws from the superior who is our neighbor the obedience that is his due.
Reply to Objection 1. The definition given by Ambrose refers to mortal sin, which has the character of perfect sin. Venial sin is not disobedience, because it is not contrary to a precept, but beside it. Nor again is every mortal sin disobedience, properly and essentially, but only when one contemns a precept, since moral acts take their species from the end. And when a thing is done contrary to a precept, not in contempt of the precept, but with some other purpose, it is not a sin of disobedience except materially, and belongs formally to another species of sin.
Reply to Objection 2. Vainglory desires display of excellence. And since it seems to point to a certain excellence that one be not subject to another's command, it follows that disobedience arises from vainglory. But there is nothing to hinder mortal sin from arising out of venial sin, since venial sin is a disposition to mortal.
Reply to Objection 3. No one is bound to do the impossible: wherefore if a superior makes a heap of precepts and lays them upon his subjects, so that they are unable to fulfil them, they are excused from sin. Wherefore superiors should refrain from making a multitude of precepts.
Source: Summa Theologica (New Advent)