Abel followed God's specific instructions regarding the acceptability of the offering; these instructions were evidently made known to all of Adam's family. Cain would have received the same instructions - requesting a first-born animal sacrifice, symbolizing the need for a savior to atone for our sins. Perhaps Cain, knowing these sacrificial instructions as well as the specific prophesies about Eve's offspring (Cain was Eve's firstborn) bruising Satan, may have envisioned himself as the Messiah figure, having no sense of his own sinful nature and the need for a savior. Cain essentially demanded religion on his own terms; we dare not follow in his footsteps.