What was there about Cain’s offering that made it unacceptable to God? Was it the offering itself, or was it Cain’s attitude?
It would appear that Cain was at fault, both in his attitude and in the offering he presented to the Lord. Cain’s sacrifice consisted of crops he had raised in his garden (Gen. 4:3), rather than a blood sacrifice, as his younger brother Abel had set before the Lord.
That Abel presented a blood sacrifice and did so in faith (cf. Heb. 11:4) strongly suggests that he was claiming a divine promise of grace as he laid his lamb on the altar—a promise he had learned from his parents. God therefore was pleased with Abel’s offering (Gen. 4:4) and responded to him with approval, in contradistinction to His rejection of Cain’s offering. It would seem that Cain had followed his own judgment in choosing a bloodless sacrifice, disregarding the importance of blood as explained by God to Adam and Eve, and disregarding the principle of substitutionary atonement that later found its complete fulfillment in the crucifixion of Christ.
Cain’s willful substitution of the work of his own hands in place of atoning grace was followed by a savage jealousy and burning resentment toward his younger brother (Gen. 4:5). This eventuated in his murder of Abel out in the field, where Cain supposed no one could see him. His proud self-will led him to commit homicide, and his descendants carried on something of his man-centered, God-denying attitude for many generations to come (see Gen. 4:18–24; cf. “the daughters of men” in Gen. 6:2).