3 For at the beginning God had respect to the gifts of Abel, because he offered with single-mindedness and righteousness; but He had no respect unto the offering of Cain, because his heart was divided with envy and malice, which he cherished against his brother, as God says when reproving his hidden [thoughts], “Though you offer rightly, yet, if you do not divide rightly, have you not sinned? Be at rest;” since God is not appeased by sacrifice. For if any one shall endeavour to offer a sacrifice merely to outward appearance, unexceptionably, in due order, and according to appointment, while in his soul he does not assign to his neighbour that fellowship with him which is right and proper, nor is under the fear of God—he who thus cherishes secret sin does not deceive God by that sacrifice which is offered correctly as to outward appearance; nor will such an oblation profit him anything, but [only] the giving up of that evil which has been conceived within him, so that sin may not the more, by means of the hypocritical action, render him the destroyer of himself. Wherefore did the Lord also declare: “Woe unto you, scribes and Pharisees, hypocrites, for you are like whited sepulchres. For the sepulchre appears beautiful outside, but within it is full of dead men's bones, and all uncleanness; even so you also outwardly appear righteous unto men, but within you are full of wickedness and hypocrisy.” For while they were thought to offer correctly so far as outward appearance went, they had in themselves jealousy like to Cain; therefore they slew the Just One, slighting the counsel of the Word, as did also Cain. For [God] said to him, “Be at rest;” but he did not assent. Now what else is it to “be at rest” than to forego purposed violence? And saying similar things to these men, He declares: “You blind Pharisee, cleanse that which is within the cup, that the outside may be clean also.” And they did not listen to Him. For Jeremiah says, “Behold, neither your eyes nor your heart are good; but [they are turned] to your covetousness, and to shed innocent blood, and for injustice, and for man-slaying, that you may do it.” And again Isaiah says, “You have taken counsel, but not of Me; and made covenants, [but] not by My Spirit.” In order, therefore, that their inner wish and thought, being brought to light, may show that God is without blame, and works no evil — that God who reveals what is hidden [in the heart], but who works not evil— when Cain was by no means at rest, He says to him: “To you shall be his desire, and you shall rule over him.” Thus did He in like manner speak to Pilate: “You should have no power at all against Me, unless it were given you from above;” God always giving up the righteous one [in this life to suffering], that he, having been tested by what he suffered and endured, may [at last] be accepted; but that the evildœr, being judged by the actions he has performed, may be rejected. Sacrifices, therefore, do not sanctify a man, for God stands in no need of sacrifice; but it is the conscience of the offerer that sanctifies the sacrifice when it is pure, and thus moves God to accept [the offering] as from a friend. “But the sinner,” says He, “who kills a calf [in sacrifice] to Me, is as if he slew a dog.”
Source: Adversus haereses (New Advent)