From the Dominican Walter Farrell, as quoted in Donald Connolly’s Renewing Your Faith:
The catechism defines sin as a thought, word, deed or omission against the law of God.” But the word “omission” is a little unfortunate. It has the air of the accidental about it, like forgetting to take medicine or absent-mindedly going out without an umbrella. Actually, sin is impossible without some positive act back along the road from which that sin has come. Sins do not just happen, they are willed; they are not accidents that stain our souls as ink might stain a table-cloth; we must deliberately throw the stain at our souls. For sins are human acts, acts for which a man is responsible, which proceed under his control and to an end which he has freely chosen. Otherwise his acts, no matter how evil they may be in themselves, are not sins. So somewhere behind a sin of omission, either by way of cause, or occasion, or impediment, we must be responsible for the omission: which means that somewhere we must have willed it, whether directly or indirectly.
Yet in another sense, sins are indeed accidents. The commission of sin puts us in the position of the little boy who wants to eat green apples, but does not want the inevitable stomach-ache that goes with eating them. Nevertheless, he eats the apples. The stomach-ache is an accident so far as his will is concerned , certainly his mouth does not water in anticipation of a stomach-ache; yet in another sense he is quite willing to accept the stomach-ache as the price to be paid for eating green apples. No man wants to be a sinner, wants to turn his back upon God , wants to give up all chance for happiness and condemn himself to eternal misery. But if all that is inevitably connected with what is desired here and now, the sinner is willing to pay that price for his sin. We never quite grow up; and there is no more convincing evidence of our constant immaturity than the childish reversal of values involved in sin.
Stepping into the world of sin is like stepping into a dark tropical forest, nurtured to unbelievable growth by a sun of desire which kills healthy plants. The variety of sin rivals the variety of tropical growth, in fact surpasses it; for the variety of sin is limited only by the possibilities of a will whose limit is the infinite. It is of no use to look to that will for a distinction of the various kinds of sin; an examination of the motives of sin, meaning by motives the causes which produce sin, can tell us only that this act was or was not human, that it was or was not sin. From a terrible fear of humiliation, or from a wildly passionate love, can come the same sin of lying or murder; from the one motive of anger can come sins as widely different as blasphemy, theft and murder.
The reason for this is that sin, like every other human act, is a motion to a goal. In the world below man, we can easily determine the nature of a motion by looking either at the goal or at the active power that produced the motion; for the powers beneath man run along a determined track that leads always to the same goal. But the powers of man have no set channel along which they must necessarily flow. So, for the determination of any human act, virtuous or vicious, we must look to the goal towards which it is going, to the object of the act. to the thing desired that first set in motion that activity of a human being. In other words, the specific character of any sin, as the specific character of any virtue, its very essence, is to be judged by the object to which it is directed.
This concept, which is rooted in St. Thomas Aquinas and is certainly evident in Dante, was one that drew me to Roman Catholicism in the first place. It solved many problems that the Episcopal Church I grew up in either could or would not address. And it has protected me from the Scylla and Charbdis of both the unreasonable sentence of Reformed theology or the sloppy theodicy of modern Pentecost.