11 Again, in the case of Cain and Abel, the same proceeding is observable. For, in the first place, they set apart the fruits of their own labours to God. For we would show not from his sin only, but also from his virtue, that man was capable of knowing both these things. Wherefore that man knew sin to be an evil thing, Adam manifested; and that he knew that virtue was a good thing, Abel again made evident. For without having learned it from any one, without having heard any law promulgated respecting the first fruits, but having been taught from within, and from his conscience, he presented that sacrifice.
On this account I do not carry the argument down to a later period; but I bring it to bear upon the time of these earlier men, when there were as yet no letters, as yet no law, nor as yet prophets and judges; but Adam only existed with his children; in order that you may learn, that the knowledge of good and evil had been previously implanted in their natures. For from whence did Abel learn that to offer sacrifice was a good thing; that it was good to honour God, and in all things to give thanks?
“Why then?” replies some one, “did not Cain bring his offering?” This man also did offer sacrifice, but not in like manner. And from thence again the knowledge of conscience is apparent. For when, envying him who had been honoured, he deliberated upon murder, he conceals his crafty determination. And what says he; “Come, let us go forth into the field.” The outward guise was one thing, the pretence of love; the thought another, the purpose of fratricide. But if he had not known the design to be a wicked one, why did he conceal it?
And again, after the murder had been perpetrated, being asked of God, “Where is Abel your brother?” he answers, “I know not; Am I my brother's keeper?” Wherefore does he deny the crime? Is it not evidently because he exceedingly condemns himself. For as his father had hid himself, so also this man denies his guilt, and after his conviction, again says, “My crime is too great to obtain pardon.”
Source: Homilies on the Statues (New Advent)