9 As Cain's sacrifice of the fruit of the ground is rejected, while Abel's sacrifice of his sheep and the fat thereof is accepted, so the faith of the New Testament praising God in the harmless service of grace is preferred to the earthly observances of the Old Testament. For though the Jews were right in practising these things, they were guilty of unbelief in not distinguishing the time of the New Testament when Christ came, from the time of the Old Testament. God said to Cain, "If you offer well, yet if you divide not well, you have sinned." If Cain had obeyed God when He said, "Be content, for to you shall be its reference, and you shall rule over it," he would have referred his sin to himself, by taking the blame of it, and confessing it to God; and so assisted by supplies of grace, he would have ruled over his sin, instead of acting as the servant of sin in killing his innocent brother.
So also the Jews, of whom all these things are a figure, if they had been content, instead of being turbulent, and had acknowledged the time of salvation through the pardon of sins by grace, and heard Christ saying, "They that are whole need not a physician, but they that are sick; I came not to call the righteous, but sinners to repentance;" and, "Every one that commits sin is the servant of sin;" and, "If the Son make you free, you shall be free indeed," — they would in confession have referred their sin to themselves, saying to the Physician, as it is written in the Psalm, "I said, Lord, be merciful to me; heal my soul, for I have sinned against You." And being made free by the hope of grace, they would have ruled over sin as long as it continued in their mortal body.
But now, being ignorant of God's righteousness, and wishing to establish a righteousness of their own, proud of the works of the law, instead of being humbled on account of their sins, they have not been content; and in subjection to sin reigning in their mortal body, so as to make them obey it in the lusts thereof, they have stumbled on the stone of stumbling, and have been inflamed with hatred against him whose works they grieved to see accepted by God. The man who was born blind, and had been made to see, said to them, "We know that God hears not sinners; but if any man serve Him, and do His will, him He hears;" as if he had said, God regards not the sacrifice of Cain, but he regards the sacrifice of Abel.
Abel, the younger brother, is killed by the elder brother; Christ, the head of the younger people, is killed by the elder people of the Jews. Abel dies in the field; Christ dies on Calvary.
Source: Reply to Faustus the Manichaean (New Advent)