Objection 1. It would seem that a sin of omission is more grievous than a sin of transgression. For "delictum" would seem to signify the same as "derelictum" [Augustine, QQ. in Levit., qu. xx], and therefore is seemingly the same as an omission. But "delictum" denotes a more grievous offence than transgression, because it deserves more expiation as appears from Leviticus 5. Therefore the sin of omission is more grievous than the sin of transgression.
Objection 2. Further, the greater evil is opposed to the greater good, as the Philosopher declares (Ethic. viii, 10). Now to do good is a more excellent part of justice, than to decline from evil, to which transgression is opposed, as stated above (1, ad 3). Therefore omission is a graver sin than transgression.
Objection 3. Further, sins of transgression may be either venial or mortal.
But sins of omission seem to be always mortal, since they are opposed to an affirmative precept. Therefore omission would seem to be a graver sin than transgression.
Objection 4. Further, the pain of loss which consists in being deprived of seeing God and is inflicted for the sin of omission, is a greater punishment than the pain of sense, which is inflicted for the sin of transgression, as Chrysostom states (Hom. xxiii super Matth.). Now punishment is proportionate to fault. Therefore the sin of omission is graver than the sin of transgression.
On the contrary, It is easier to refrain from evil deeds than to accomplish good deeds. Therefore it is a graver sin not to refrain from an evil deed, i.e. "to transgress," than not to accomplish a good deed, which is "to omit."
I answer that, The gravity of a sin depends on its remoteness from virtue. Now contrariety is the greatest remoteness, according to Metaph. x [Didot. ed. ix, 4. Wherefore a thing is further removed from its contrary than from its simple negation; thus black is further removed from white than not-white is, since every black is not-white, but not conversely. Now it is evident that transgression is contrary to an act of virtue, while omission denotes the negation thereof: for instance it is a sin of omission, if one fail to give one's parents due reverence, while it is a sin of transgression to revile them or injure them in any way. Hence it is evident that, simply and absolutely speaking, transgression is a graver sin than omission, although a particular omission may be graver than a particular transgression.
Reply to Objection 1. "Delictum" in its widest sense denotes any kind of omission; but sometimes it is taken strictly for the omission of something concerning God, or for a man's intentional and as it were contemptuous dereliction of duty: and then it has a certain gravity, for which reason it demands a greater expiation.
Reply to Objection 2. The opposite of "doing good" is both "not doing good," which is an omission, and "doing evil," which is a transgression: but the first is opposed by contradiction, the second by contrariety, which implies greater remoteness: wherefore transgression is the more grievous sin.
Reply to Objection 3. Just as omission is opposed to affirmative precepts, so is transgression opposed to negative precepts: wherefore both, strictly speaking, have the character of mortal sin. Transgression and omission, however, may be taken broadly for any infringement of an affirmative or negative precept, disposing to the opposite of such precept: and so taking both in a broad sense they may be venial sins.
Reply to Objection 4. To the sin of transgression there correspond both the pain of loss on account of the aversion from God, and the pain of sense, on account of the inordinate conversion to a mutable good. On like manner omission deserves not only the pain of loss, but also the pain of sense, according to Matthew 7:19, "Every tree that bringeth not forth good fruit shall be cut down, and shall be cast into the fire"; and this on account of the root from which it grows, although it does not necessarily imply conversion to any mutable good.
Source: Summa Theologica (New Advent)