Cardinal Manning, commenting on St. John’s statement that “all iniquity is sin” (1 Jn 5:17), explains that “iniquity means all departure from the rectitude of God and of the law of God. Iniquity is inequality, or crookedness. Everything that is not conformed to the rectitude of God, to His perfections, to His law, and to His will, is sin.”
Then, following upon St. John’s distinction between “a sin unto death” and “a sin which is not unto death” (1 Jn 5:16), Cardinal Manning defines the elements that constitute mortal sin, thereby distinguishing it from venial sin. He explains: “To constitute a mortal sin it is necessary that the man who commits it should know what he does—there must be a knowledge of the intellect; if not, the sin is only, as I then said, a material sin, and not a formal sin, unless his ignorance be a culpable and guilty ignorance. Next, he must not only know that he is doing wrong, but his will must consent to the wrong-doing. Thirdly, he must know and consent deliberately, with such an advertence or attention to what he is about as to make him conscious of his action.”
He observes: “A man who should transgress the law of God in the least possible way would fulfil these three conditions. It would be a transgression of the law of God if I should take an apple off the tree of my neighbour without his leave. It was his: I had not a right to take it, and I thereby broke the commandment, ‘Thou shalt not steal’; but that certainly would not be a sin unto death. It became a sin unto death when a divine prohibition was laid upon such an act under pain of death, and that the pain of eternal death; but where there is no such command laid under pain of death, it is quite clear that the taking of an apple would not constitute a sin unto death. Therefore it is necessary that there should be a gravity in the matter of the sin.”
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).