As the Hebrews went forth from Egypt with the moral law written on their hearts to receive it engraven upon stone, so they entered the wilderness with the vague feeling that their God was to be approached by oblations — to receive in that wilderness a minute and elaborate code of sacrificial laws to be executed by a divinely-appointed priesthood.
The nature of the patriarchal sacrifices is still a question among theologians. Orthodox polemics generally deem it incumbent on them to demonstrate the expiatory character of these sacrifices, while the rationalistic school quite unanimously deny this as an unwarrantable assumption. Several evangelical writers take the same view. To neither party is there scriptural ground for dogmatism, for the sacred oracles are silent respecting the origin and nature of the early sacrificial offerings. Hence they go beyond the sacred record, who, in their zeal for orthodoxy, inform us that Cain’s sacrifice was rejected because there was no blood in it, betokening his need of the death of another as a satisfaction for his sin, while Abel’s was accepted because it had that vital element, rendering it pleasing to his Creator. Sacred history not only contains no such declaration, but it plainly intimates another cause for the difference between the two offerings. God expostulates with the wrathful fratricide, and explicitly declares that the imperfection of his offering lies in the moral state of the offerer: “If thou doest well, shalt thou not be accepted?” In Hebrews 11:4 the writer declares that Abel’s acceptableness was because of his faith, leaving us to infer that the lack of this element was the radical defect in Cain’s oblation.
— Commentary on Leviticus.