Hi Endoscopy, simple answer, many bible students believe that Cain's offering is the works of his hands and not the blood sacrifice that God requires for the forgiveness of sin's. Able's offering was by grace through faith but Cain's offering was by works not grace. Dcon said he will be gone for a couple of days, if my answer is insufficient he will set things straight.
Genesis4:4-5
4b) The Lord looked with favor on Abel and his offering, 5) but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Hebrews11:4
4) By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks.
Hebrews9:22
22) According to the Law, in fact, nearly everything must be purified with blood, and without the shedding of blood there is no forgiveness.
Genesis4:4-5
4b) The Lord looked with favor on Abel and his offering, 5) but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Hebrews11:4
4) By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks.
Hebrews9:22
22) According to the Law, in fact, nearly everything must be purified with blood, and without the shedding of blood there is no forgiveness.
We know in genesis that God said about crushing the head of the one who bruised his heel.
Would Adam & Eve knew what that it meant?
What do you think?