Objection 1. It would seem that every sin includes an action. For as merit is compared with virtue, even so is sin compared with vice. Now there can be no merit without an action. Neither, therefore, can there be sin without action.
Objection 2. Further, Augustine says (De Lib. Arb. iii, 18) [Cf. De Vera Relig. xiv.]: So "true is it that every sin is voluntary, that, unless it be voluntary, it is no sin at all." Now nothing can be voluntary, save through an act of the will. Therefore every sin implies an act.
Objection 3. Further, if sin could be without act, it would follow that a man sins as soon as he ceases doing what he ought. Now he who never does something that he ought to do, ceases continually doing what he ought. Therefore it would follow that he sins continually; and this is untrue. Therefore there is no sin without an act.
On the contrary, It is written (James 4:17): "To him... who knoweth to do good, and doth it not, to him it is a sin." Now "not to do" does not imply an act. Therefore sin can be without act.
I answer that, The reason for urging this question has reference to the sin of omission, about which there have been various opinions. For some say that in every sin of omission there is some act, either interior or exterior--interior, as when a man wills "not to go to church," when he is bound to go--exterior, as when a man, at the very hour that he is bound to go to church (or even before), occupies himself in such a way that he is hindered from going. This seems, in a way, to amount to the same as the first, for whoever wills one thing that is incompatible with this other, wills, consequently, to go without this other: unless, perchance, it does not occur to him, that what he wishes to do, will hinder him from that which he is bound to do, in which case he might be deemed guilty of negligence. On the other hand, others say, that a sin of omission does not necessarily suppose an act: for the mere fact of not doing what one is bound to do is a sin.
Now each of these opinions has some truth in it. For if in the sin of omission we look merely at that in which the essence of the sin consists, the sin of omission will be sometimes with an interior act, as when a man wills "not to go to church": while sometimes it will be without any act at all, whether interior or exterior, as when a man, at the time that he is bound to go to church, does not think of going or not going to church.
If, however, in the sin of omission, we consider also the causes, or occasions of the omission, then the sin of omission must of necessity include some act. For there is no sin of omission, unless we omit what we can do or not do: and that we turn aside so as not to do what we can do or not do, must needs be due to some cause or occasion, either united with the omission or preceding it. Now if this cause be not in man's power, the omission will not be sinful, as when anyone omits going to church on account of sickness: but if the cause or occasion be subject to the will, the omission is sinful; and such cause, in so far as it is voluntary, must needs always include some act, at least the interior act of the will: which act sometimes bears directly on the omission, as when a man wills "not to go to church," because it is too much trouble; and in this case this act, of its very nature, belongs to the omission, because the volition of any sin whatever, pertains, of itself, to that sin, since voluntariness is essential to sin. Sometimes, however, the act of the will bears directly on something else which hinders man from doing what he ought, whether this something else be united with the omission, as when a man wills to play at the time he ought to go to church--or, precede the omission, as when a man wills to sit up late at night, the result being that he does not go to church in the morning. In this case the act, interior or exterior, is accidental to the omission, since the omission follows outside the intention, and that which is outside the intention is said to be accidental (Phys. ii, text. 49,50). Wherefore it is evident that then the sin of omission has indeed an act united with, or preceding the omission, but that this act is accidental to the sin of omission.
Now in judging about things, we must be guided by that which is proper to them, and not by that which is accidental: and consequently it is truer to say that a sin can be without any act; else the circumstantial acts and occasions would be essential to other actual sins.
Reply to Objection 1. More things are required for good than for evil, since "good results from a whole and entire cause, whereas evil results from each single defect," as Dionysius states (Div. Nom. iv): so that sin may arise from a man doing what he ought not, or by his not doing what he ought; while there can be no merit, unless a man do willingly what he ought to do: wherefore there can be no merit without act, whereas there can be sin without act.
Reply to Objection 2. The term "voluntary" is applied not only to that on which the act of the will is brought to bear, but also to that which we have the power to do or not to do, as stated in Ethic. iii, 5. Hence even not to will may be called voluntary, in so far as man has it in his power to will, and not to will.
Reply to Objection 3. The sin of omission is contrary to an affirmative precept which binds always, but not for always. Hence, by omitting to act, a man sins only for the time at which the affirmative precept binds him to act.
Source: Summa Theologica (New Advent)