Why did God accept Abel's sacrifice and not Cain's?  This lesson explains, and it is vital to understand the answer!  Cain's problem is the problem of religious people all over the world who attempt to approach God "the way of Cain" (Jude 11), which is by their own works (fruit).  What made Abel's offering "a more excellent sacrifice"?  It pointed to Christ.

Isn't it interesting that Cain, who was too "dignified" to offer a blood sacrifice, quickly and angrily shed the blood of his own brother? Cain's life should be a strong lesson about the danger in trusting human "reason" rather than divine revelation; of continuing in human willfulness opposed to divine will; of human pride versus Christlike humility; of sinking to hatred rather than rising to love; of presenting fleshly excuses instead of seeking divine grace.  Wrong choices end in human loneliness instead of enjoying divine fellowship.  To be without God is the worst thing in this life, but it is worse in the life to come.